Renamed client-side admin url helper
Renamed client-side admin url helper

fixes #2254

- includes test

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