Move setup to API
Move setup to API

closes #3136
- moved setup to authentication API
- added `POST /ghost/api/v0.1/authentication/setup` to execute the
setup process
- added `GET /ghost/api/v0.1/authentication/setup` to check if blog is
already set up (needed for #3145)
- removed unused methods from api/users.js

TryGhost/Ghost,TryGhost/Ghost,TryGhost/Ghost