Complete moveover to new Notification API format

fixes #2775
- Fix all occurences of notifications.add to use proper API format
Merge pull request #2970 from halfdan/2775-notification-fix

Complete moveover to new Notification API format
Fix slug create and update logic for posts

-Remove unnecessary calls to the slug API endpoint on the
 content screen.
-Only make slug API endpoint calls in editor when the post is new.
-Improve logic for when to update the slug.
Merge pull request #2968 from jaswilli/slugs

Fix slug create and update logic for posts
[WIP] Functional tests for Ember

Main things outstanding:

- The ember editor doesn't work under test conditions at present
- Need to finish converting the settings tests