Redirect user if signup invitation isn't valid
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- If the invitation is no longer valid (or didn’t exist in the first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.

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