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

Reduce progress bar noise during brakeman run

See merge request !3101

Properly display YAML front matter in Markdown

See merge request !3072

This filter will detect YAML Front Matter and convert it to an HTML
table for prettier formatting.

Add a spec for WikiPipeline

Removes the specs from GollumTagsFilter that were more like integration
tests for the pipeline than unit tests of the filter.

See merge request !3054