Sidekiq memory killer

This change adds an optional Sidekiq memory killer middleware that
will perform a graceful shutdown when the Sidekiq process is using
too much memory. When combined with a process supervision daemon
(such as Runit in omnibus-gitlab) this should provide Good Enough
handling of the Sidekiq memory leaks we are seeing.

The memory killer is off by default.

See merge request !1276

When enabled, this middleware allows Sidekiq to detect that its RSS has
exceeded a maximum value, triggering a graceful shutdown. This
middleware should be combined with external process supervision that
will restart Sidekiq after the graceful shutdown, such as Runit.

Add Dockerfile to build an Omnibus GitLab image

See merge request !244

Office analogy

introduced office analogy to architecture.md

See merge request !1275

add details on protected branches

Add some details from https://about.gitlab.com/2014/11/26/keeping-your-code-protected/:

Who can:
* Force push to non-protected branches
* Force push to protected branches 
* Remove protected branches

Select a tag

Related to #1603.

Prevent users looking to install GitLab from source from installing a release candidate by mistake.

See merge request !1274

Update time zone rake task for production. Resolves #8387

Conflicts:
	GITLAB_SHELL_VERSION

Security release ownership

We saw the following problems around the 7.4.4 release:

- the fix got merged into master while it should have been held back
- there was a security fix but no release manager to release it

With this change we want to improve our process and documentation
to prevent situations like this in the future.

See merge request !1273

Backup cron mode

If you have your server configured to receive emails containing the output of
cron jobs it is annoying to get long spammy emails from the backup script. This
change adds a 'cron mode' that makes the backup script silent unless something
goes wrong during the backup.

See merge request !1268

start gitlab after mysql tweaks

Add missing timestamps to the 'members' table

In 20140914145549_migrate_to_new_members_model.rb we forgot to set the
created_at and updated_at times for new records in the 'members' table. This
became a problem after commit c8e78d97 which
was added in GitLab 7.5. With this migration we ensure that all rows in
'members' have at least some created_at and updated_at timestamp.

See merge request !1272

Possibility to create Milestones or Labels when Issues are disabled

https://dev.gitlab.org/gitlab/gitlabhq/issues/1682

See merge request !1270

Add GitLab shell update to update guide

See merge request !1269