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
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)

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

## 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
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 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
Updated dropdown CSS
This is a start on #3279 by updating the CSS for the dropdown menu. This doesnt include any CSS for the title bar or filter field

See merge request !3092