Small Note code cleanup

Fix link to wiki page in wiki comment notification

Improve development experience.

Fix notification for notes on a wall. (regression)

The fact that Notes on Walls have a noteable_type value of "nil" got
lost in the refactoring of the observer.

Fix displaying events for deleted things on the dashboard (again)

Improve (static) suggestions in global search field

Hooks API (List one hook & edit)

Fixed mistake with updating project hook

Reduce complexity: replace case statement with method lookup.

Also (I think) I found few mistakes with snippets requests.

Added documentation for getting project hook and editing project hook.

Added methods for listing one project hook and editing hooks.
GET /project/:id/hooks/:hook_id
PUT /project/:id/hooks/:hook_id

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