Expose User#last_sign_in_at and User#confirmed_at for admins

Closes #840

See merge request !3090

Merge branch 'improve/3102-memoize-group-instance-variable-in-admin-groups-controller' into 'master'

Remove instance variable `@group` multiple assignment

See merge request !3127

Remove duplicate 'cache_store' configuration in test environment

[ci skip]

See merge request !3061

Added basic SQL guidelines

This is mostly based on the various changes I've made over the past few months.

See merge request !3091

update slack instructions

the previous instructions no longer apply, there is no "Incoming WebHooks" row anymore.

luckily there exists direct link to web hooks, so the previous steps aren't even neccessary

See merge request !3052

add link to web hooks docs

Add link to web hooks docs on custom hooks docs page.

See merge request !3082

adds language names to projects list [image attached]

See merge request !3000

[ci skip]

Deprecated GitLab CI API clean up

Deprecated GitLab CI API clean up.

The intent here is to clean up deprecated GitLab CI documentation leaving only relevant information.
Since we merged `Ci::Project` to `Project` most of this documentation is outdated.

Closes #13610 

See merge request !3003

add bundle clean to upgrade docs

This MR adds a bundle clean after installing new gems. Bundle clean removes old, not needed gems to keep the system clean.

See merge request !2809

Added Troubleshooting information for most used services.

This replaces the changes started at gitlab-com/www-gitlab-com!1603

I'd like to add similar troubleshooting information for EE only services, but that needs to happen on a different MR on the EE side. Probably after this one gets merged.

cc @ernstvn @dblessing @balameb @cabargas @kelvinmutuma 

See merge request !3080

Document changes to the initial admin password

Addresses https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3068#note_4117550

[ci skip]

See merge request !3120

[ci skip]

Remove redundant integration tests

These three tests were essentially checking that adding a note to
something updated its `noteable`'s `updated_at` attribute.

This is well-tested Rails behavior and we shouldn't feel the need to
write an integration test to verify it. At most we should be ensuring
that the association definition adds the `touch: true` option, which we
now do in Note's unit test.

See merge request !3117

Set cache headers for raw blobs

This changes allows browsers and (in the case of public projects)
proxy caches to cache raw Git blob responses.

See merge request !3113

[ci skip]

removes automatic setting of main_language to project for it to set a main language you have now to make a commit to the project

These three tests were essentially checking that adding a note to
something updated its `noteable`'s `updated_at` attribute.

This is well-tested Rails behavior and we shouldn't feel the need to
write an integration test to verify it. At most we should be ensuring
that the association definition adds the `touch: true` option, which we
now do in Note's unit test.

Labels should be visible in dashboard and group milestone views

Closes #13621

See merge request !2931

Add support for cross project references for labels

## Summary

Support for cross project references for labels.

## Rationale

1.   Cross project label references are currently not supported in GitLab
1.   `to_reference` method signature in `Label` model breaks the abstraction introduced in `Referable`.

      `concerns/referable.rb:  def to_reference(_from_project = nil)`

      Signatures:

      ```
      label.rb:           def to_reference(format = :id)

      commit_range.rb:    def to_reference(from_project = nil)
      commit.rb:          def to_reference(from_project = nil)
      external_issue.rb:  def to_reference(_from_project = nil)
      group.rb:           def to_reference(_from_project = nil)
      issue.rb:           def to_reference(from_project = nil)
      merge_request.rb:   def to_reference(from_project = nil)
      milestone.rb:       def to_reference(from_project = nil)
      project.rb:         def to_reference(_from_project = nil)
      snippet.rb:         def to_reference(from_project = nil)
      user.rb:            def to_reference(_from_project = nil)
      ```

     This MR suggests using `def to_reference(from_project = nil, format: :id)` which makes use of keyword arguments and preserves abstract interface.

1.   We need support for cross project label references when we want to move issue to another project

     It may happen that issue description, system notes or comments contain reference to label and this reference will be invalid after moving issue to another project and will not be displayed correctly unless we have support for cross project references.

     Merge request that needs this feature: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2831


I think that cross project label references may be useful, (example: `Hey, see our issues for CI in GitLab CE! - gitab-org/gitlab-ce~"CI"`).

cc @JobV @DouweM @rspeicher 

See merge request !2966