Improve performance of user profiles, finding groups, and finding projects

This MR improves the following:

* Rendering of profile pages and Atom feeds
* Finding groups (using GroupsFinder & friends)
* Finding projects (using ProjectsFinder & friends)

Initially this MR was intended to only improve rendering of Atom feeds, but over time other fixes were introduced as well as the same code was the cause of all these problems.

See merge request !1790

Check which folders and archives should be packed before passing to tar command.

If user uses backup task with SKIP and skips one of the archives listed(uploads, builds, artifacts) backup create will give an error: `Cannot stat: No such file or directory`.

This MR fixes that by checking for skipped items.
Additionally, compact everything to avoid `TypeError: no implicit conversion of nil into String` errors.

See merge request !1824

Fix huge line height for diff files list
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1826

Backport relevant changes from gitlab-org/gitlab-ee!51

To do:

- [x] Update gitlab-shell

See merge request !1822

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

When getting the projects of a user we should get the public _and_
internal projects, not just the public ones.

Apparently MySQL doesn't support this syntax:

    (...) UNION (...)

instead it only supports:

    ...
    UNION
    ...

This won't work efficiently if you happen to have a lot of projects.

These changes are based on those from commit
03f5ff75, except they use a UNION
instead of plucking IDs into memory.

Previously this controller would in multiple places load tons (read:
around 65000) project and/or group IDs into memory. These changes in
combination with the previous commits significantly cut down loading
times of user profile pages and the Atom feeds of users.

These methods no longer include public groups/projects (that don't
belong to the actual user) as this is handled by the various finder
classes now. This also removes the need for passing extra arguments.

Note that memoizing was removed _explicitly_. For whatever reason doing
so messes up the users controller to a point where it claims a certain
user does _not_ have access to certain groups/projects when it does have
access. Existing code shouldn't be affected as these methods are only
called in ways that they'd run queries anyway (e.g. a combination of
"any?" and "each" which would run 2 queries regardless of memoizing).

This method can be used to filter projects to those visible to a given
user.

These methods will be used to get a list of groups, optionally
restricted to only those visible to a given user.

This will be used to move some querying logic from the users controller
to the Event model (where it belongs).

This class now uses a UNION (when needed) instead of plucking tens of
thousands of project IDs into memory. The tests have also been
re-written to ensure all different use cases are tested properly
(assuming I didn't forget any cases).

The finder has also been broken up into 3 different finder classes:

* ContributedProjectsFinder: class for getting the projects a user
  contributed to.
* PersonalProjectsFinder: class for getting the personal projects of a
  user.
* ProjectsFinder: class for getting generic projects visible to a given
  user.

Previously a lot of the logic of these finders was handled directly in
the users controller.

In the previous setup the GroupsFinder class had two distinct tasks:

1. Finding the projects user A could see
2. Finding the projects of user A that user B could see

Task two was actually handled outside of the GroupsFinder (in the
UsersController) by restricting the returned list of groups to those the
viewed user was a member of. Moving all this logic into a single finder
proved to be far too complex and confusing, hence there are now two
finders:

* GroupsFinder: for finding groups a user can see
* JoinedGroupsFinder: for finding groups that user A is a member of,
  restricted to either public groups or groups user B can also see.

This new setup no longer loads any IDs into memory using "pluck",
instead using SQL UNIONs to merge the various datasets together. This
results in greatly improved query performance as well as a reduction of
memory usage.

The old setup was in particular problematic when requesting the
authorized projects _including_ public/internal projects as this would
result in roughly 65000 project IDs being loaded into memory. These IDs
would in turn be passed to other queries.

This removes the need for plucking any IDs into Ruby.

By moving the default sort order into a separate scope (and calling this
from the default scope) we can more easily re-apply a default order
without having to specify the exact column/ordering all over the place.

This allows retrieving of the list of authorized projects using a single
query, without having to load any IDs into Ruby. This in turn also means
we can remove the method User#authorized_projects_id.

This class can be used to join multiple AcitveRecord::Relation objects
together using a SQL UNION statement. ActiveRecord < 5.0 sadly doesn't
support UNION and existing Gems out there don't handle prepared
statements (e.g. they never incremented the variable bindings).

Instead of using MAX(events.updated_at) we can simply sort the events in
descending order by the "id" column and grab the first row. In other
words, instead of this:

    SELECT max(events.updated_at) AS max_id
    FROM events
    LEFT OUTER JOIN projects   ON projects.id   = events.project_id
    LEFT OUTER JOIN namespaces ON namespaces.id = projects.namespace_id
    WHERE events.author_id IS NOT NULL
    AND events.project_id IN (13083);

we can use this:

    SELECT events.updated_at AS max_id
    FROM events
    LEFT OUTER JOIN projects   ON projects.id   = events.project_id
    LEFT OUTER JOIN namespaces ON namespaces.id = projects.namespace_id
    WHERE events.author_id IS NOT NULL
    AND events.project_id IN (13083)
    ORDER BY events.id DESC
    LIMIT 1;

This has the benefit that on PostgreSQL a backwards index scan can be
used, which due to the "LIMIT 1" will at most process only a single row.
This in turn greatly speeds up the process of grabbing the latest update
time. This can be confirmed by looking at the query plans. The first
query produces the following plan:

    Aggregate  (cost=43779.84..43779.85 rows=1 width=12) (actual time=2142.462..2142.462 rows=1 loops=1)
      ->  Index Scan using index_events_on_project_id on events  (cost=0.43..43704.69 rows=30060 width=12) (actual time=0.033..2138.086 rows=32769 loops=1)
            Index Cond: (project_id = 13083)
            Filter: (author_id IS NOT NULL)
    Planning time: 1.248 ms
    Execution time: 2142.548 ms

The second query in turn produces the following plan:

    Limit  (cost=0.43..41.65 rows=1 width=16) (actual time=1.394..1.394 rows=1 loops=1)
      ->  Index Scan Backward using events_pkey on events  (cost=0.43..1238907.96 rows=30060 width=16) (actual time=1.394..1.394 rows=1 loops=1)
            Filter: ((author_id IS NOT NULL) AND (project_id = 13083))
            Rows Removed by Filter: 2104
    Planning time: 0.166 ms
    Execution time: 1.408 ms

According to the above plans the 2nd query is around 1500 times faster.
However, re-running the first query produces timings of around 80 ms,
making the 2nd query "only" around 55 times faster.

Fix github importer to handle empty issues