Allow user to accept invitation

closes #3081
- added route `/ghost/api/v0.1/authentication/invitation`
- added accept invitation
- added signup with token
- removed check() from users api
- fixed promise in resetPassword()
AHHH I've got you. Remove add, and rename invite *to* add. If I had thought about it, that would've been the only thing that made sense, but, *thinking*?  Meh.