[CE backport] Saved configuration for issue board

See merge request gitlab-org/gitlab-ce!15009

Use the correct project visibility in system hooks

Closes #39496

See merge request gitlab-org/gitlab-ce!15065

grab the correct username when confirming secondary email

Closes #39366

See merge request gitlab-org/gitlab-ce!15010

Remove Autosave class from global namespace

See merge request gitlab-org/gitlab-ce!14856

Update docs on creating MRs

See merge request gitlab-org/gitlab-ce!14982

Fetch the merged branches at once.

Checking it one by one in the view. We don't cache this yet
because this would already much improve the performance.

A naive test against a particularly large repository:

``` ruby
begin
  now = Time.now
  branches.map{ |b| r.merged_to_root_ref?(b.name) }
  Time.now - now
end # 8.265830782
```

Around 10 times faster:

``` ruby
begin
  now = Time.now
  r.merged_branches(branches.map(&:name))
  Time.now - now
end # 0.807405397
```

This should make the branches page usable.

See merge request gitlab-org/gitlab-ce!14729

Move doc/development/ee_features.md to CE

See merge request gitlab-org/gitlab-ce!15066

Cache commits on the repository model

See merge request gitlab-org/gitlab-ce!14863

Avoid using Rugged in Gitlab::Git::Wiki#preview_slug

See merge request gitlab-org/gitlab-ce!15054

Add configurable option to display username in user avatar link component

See merge request gitlab-org/gitlab-ce!14902

Sanitize k8s default_namespace

Closes #38692

See merge request gitlab-org/gitlab-ce!15053

Revert "Merge branch '36670-remove-edit-form' into 'master'"

Closes #39441

See merge request gitlab-org/gitlab-ce!15049

Change default disabled merge request widget message to be more general

Closes #39188

See merge request gitlab-org/gitlab-ce!14960

Now, when requesting a commit from the Repository model, the results are
not cached. This means we're fetching the same commit by oid multiple times
during the same request. To prevent us from doing this, we now cache
results. Caching is done only based on object id (aka SHA).

Given we cache on the Repository model, results are scoped to the
associated project, eventhough the change of two repositories having the
same oids for different commits is small.

Semi-linear history merge is now available in CE.

See merge request gitlab-org/gitlab-ce!15059

Remove groups_select from global namespace & simplifies the code

See merge request gitlab-org/gitlab-ce!15008

Fix https://gitlab.com/gitlab-org/gitlab-ce/issues/39188

Already mentioned in the list above this one.