User can send message to email not in system

Doing so creates an invitation associated with the message
Invitation sends email on create
Use fixture_builder
Find invitations by token
Invitation mailers include a link
Add RSVP model
Add responded_to field to invitations
Update messages and invitations when RSVPing
Build RSVPs with a user and an invitation
Add RSVPs controller
Add NoChat header
Nix password confirmation
Add invitations/show
Accept outstanding invitations when creating user
Show appropriate copy for claimed invites
Update invitation e-mail copy