Step 1:
* Add milestone_id in the model for merge_requests
* Make it possible to create or update a Merge Request with an attached
  Milestone detail
* Add the possibility to filter by Milestone and / or Assignee in the
  Merge Requests listing page

* Refactor and document methods
* Rename merge to merge!
* Fixes #1544

Merge branch 'master' into non-interactive-aws-install

Conflicts:
	doc/installation.md

Fix merge mess in installation.md

Didn't bother with files in db/, config/, or features/

Any associations, validations, delegates, scopes and methods that
were exactly the same in both Issue and MergeRequest models have been
moved to a new IssueCommonality module (role) that gets included by each
class.

There was actually quite a bit of duplication, because MergeRequests are
basically just specialized Issues.