Refactor tree views and improve consistency

Remove redundant branch labels from MR form

Fixes #1525

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)