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

Indicate when an MR diverged from the target branch

This adds an indicator to the "Merge MR" box, to tell if and how much an MR diverged from its target branch.

For instance, consider an MR to merge the branch `feature` into `master`. Some other commits were added to `master` since `feature` was created, and the two branches diverged.

```text
o master
|
o    o feature
|    |
o    o
|  /
o
```

In this case, there will be a label in the MR Merge box stating:

> This MR is by 3 commits behind the target branch `master`.

## Screenshots

### The branch diverged from the target (UI Proposal)

![UI_suggestion_1](/uploads/cd5bee3959e68026ec7d5097259d53f4/UI_suggestion_1.png)

### The branch diverged from the target (alternative UI Proposal)

![UI_suggestion_2](/uploads/f36977101b59a610850e129837dfbc83/UI_suggestion_2.png)

## How is this useful?

- In a _rebase-workflow_ (MR are preferably rebased before being merged), the reviewer wants to know if an MR is rebased on the target branch before merging it. 
    
    _With this indicator, the reviewer knows immediately if the branch is rebased, or if she needs to ask the committer to rebase its branch._

<br>

- To keep the git history readable, a team prefers to avoid merging branches that really lag a lot behind the target branch. Merging an MR that is 10 commits behind is fine, but 200 is too much.

    _With this indicator, the reviewer can see on the MR page if the branch is really far behind the target – or only a few commits behind._

## Open questions

We've been using this at @captaintrain for a few months now, and found it quite useful.

I guess the open-questions are mostly: what UI would be the more adequate? Any thoughts on this, on the general usefulness and/or on the code?

See merge request !2217

# Conflicts:
#	app/models/note.rb

Remove unused DiffLine model

Added in Mar 2014. Only usage removed in Sep 2014.

See merge request !3105

Make better use of the `Note#for_*?` methods

We've got all these polymorphic type-checking methods, might as well use
'em everywhere it makes sense.

See merge request !3106

Minor Note model cleanup

Remove `Note#noteable_type_name`

This method was added in Dec 2012, its single use was removed two days
later, and it's been there ever since.

---

Remove `Note#system?` "override"

This method was duplicating default Rails behavior.

See merge request !3104

Add a TL;DR version in quickstart guide



See merge request !3026

We've got all these polymorphic type-checking methods, might as well use
'em everywhere it makes sense.

Added in Mar 2014. Only usage removed in Sep 2014.

Reduce progress bar noise during brakeman run

See merge request !3101

This method was duplicating default Rails behavior.

This method was added in Dec 2012, its single use was removed two days
later, and it's been there ever since.

Upgrade carrierwave to 0.10.0.

Closes #2805 

See merge request !3084

Issue sidebar mobile

Fixes #13592 
Fixes #14037 

See merge request !3050

Properly display YAML front matter in Markdown

See merge request !3072

Allow the initial admin to set a password

Closes #1980

See merge request !3068

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

Closes #1980

Factory cleanup

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/5880

See merge request !2847

Move `update_merge_request` ability to Developer

Closes #13663

See merge request !3099

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

In UsersController#calendar_activities, when Date isn't parsable, fallback to Date.today

For some reason, GoogleBot accesses `/u/:username/calendar_activities` without
a `:date` param, but then the view was trying to call `#to_s(:short)` which
doesn't exist on `nil`, leading to the following Sentry report:
https://sentry.gitlap.com/gitlab/gitlabcom/issues/1182/

See merge request !3074