Move user API to primary document format
Move user API to primary document format

closes #2593

- added new format to user API methods
- changed all places where the user api was used
- updated tests and added more coverage
- little bit of cleanup in utils/api

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