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- If the invitation is no longer valid (or didn’t exist in the first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.
If we're catching just to reject, seems like we can force whoever's going to handle our promise to do all the work - ie, i don't think this catch clause is necessary