Add a `gemojione:sprite` Rake task

This task will generate a standard and Retina sprite of all of the
current Gemojione Emojis, with the accompanying SCSS map.

It will not appear in `rake -T` output, and the dependent gems are not
included in the Gemfile by default, because this task will only be
needed occasionally.

See merge request !3130

This task will generate a standard and Retina sprite of all of the
current Gemojione Emojis, with the accompanying SCSS map.

It will not appear in `rake -T` output, and the dependent gems are not
included in the Gemfile by default, because this task will only be
needed occasionally.

[ci skip]

Fix bug where Bitbucket 'closed' issues were imported as 'opened'

_Originally opened at !2930 by @iurisilvio._

- - -

Bitbucket has an undocumented status `closed`.

https://confluence.atlassian.com/bitbucket/issues-resource-296095191.html

Related to https://gitlab.com/gitlab-com/support-forum/issues/559

See merge request !2973

Remove `Snippet#expires_at`

This was removed from the interface in
https://github.com/gitlabhq/gitlabhq/pull/6027 but its implementation
lingered around for two years.

See merge request !3103

Expose User#last_sign_in_at and User#confirmed_at for admins

Closes #840

See merge request !3090

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

This was removed from the interface in
https://github.com/gitlabhq/gitlabhq/pull/6027 but its implementation
lingered around for two years.

Reduce progress bar noise during brakeman run

See merge request !3101