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