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
Merge pull request #3243 from sebgie/issue#3136

Move setup to API