Add 'invite only' site setting
Hide registration on login modal if 'invite only'
Silently fail if user tries to sneak in

When 'invite only' is enabled, there's no way for a user to create an
account unless they try and sneak in by POSTing to /users/. We will
silently fail if this happens.
Disable OmniAuth account creation if 'invite only'
Don't require authentication for invites
Merge pull request #964 from chrishunt/exclusive-club

Add 'invite only' site setting