Separate observing of Note and MergeRequests

Milestone "All Issues" filter

* Move is_assigned? and is_being_xx? methods to IssueCommonality

  This is behavior merge requests have in common with issues. Moved
  methods to IssueCommonality role. Put specs directly into
  merge_request_spec because setup differs for issues and MRs
  specifically in the "closed" factory to use.

* Add MergeRequestObserver. Parallels IssueObserver in almost every way.

  Ripe for refactoring.

* Rename MailerObserver to NoteObserver

  With merge request observing moved out of MailerObserver, all that
  was left was Note logic. Renamed to NoteObserver, added tests and
  updated application config for new observer names. Refactored
  NoteObserver to use the note's author and not rely on current_user.

* Set current_user for MergeRequestObserver

  IssueObserver and MergeRequestObserver are the only observers that
  need a reference to the current_user that they cannot look up on
  the objects they are observing.

Closes #1167

Progress on #1167

Mix commit notes with merge request notes on MR show page

Postgresql

Show numer of notes for commits and merge requests

Cleanup auto-completion and add it to all GFM inputs

Fix forward/back while browsing Tree hierarchy

* static initialization and setup moved to assets
* per request initialization moved to layout partial

Closes #1578

Improve completion of emoji and team members