[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

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

More Factory cleanup

Addresses nitpicks from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2847

See merge request !3108