Provide immediate error for invalid signup link

closes #3563
- Included server api able to check if a user record with the given
email exists
- If it does, an error message is displayed on the client (also, the
‘submit’ button is disabled
I believe this breaks some REST principals :-1: mostly by having a verb in the URL.
How about ```GET /authentication/invitation``` with either the invite key or email passed as a param?