TryGhost/Ghost — pull request #3876 by felixrieseberg (2014-08-25 02:34:51+00:00)
Title: Redirect user if signup invitation isn't valid

Closes #3565
- Added server API isInvitation (analog to isSetup), checking if an
invitation exists for a given email address.
- If the invitation is no longer valid (or didn’t exist in the first
place), the user is redirected and an error notification is shown.