Fix slug create and update logic for posts
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.

JohnONolan/Ghost-Admin,airycanon/Ghost-Admin,TryGhost/Ghost-Admin,kevinansfield/Ghost-Admin,acburdine/Ghost-Admin,TryGhost/Ghost-Admin,airycanon/Ghost-Admin,dbalders/Ghost-Admin,JohnONolan/Ghost-Admin,dbalders/Ghost-Admin,acburdine/Ghost-Admin,kevinansfield/Ghost-Admin