Implement user invites for registration

[leevigraham]

Here's my initial requirements which are up for discussion:

• Invitation code is required to register
• Invitations link the new user to a specific group in the application
• Users can be invited multiple times to multiple groups. User has many Invitations, Invitation is claimedBy a User
• There should be three ways to claim an invitation: Register, Login or if the user is already logged in a prompt (possibly requiring password) to claim.
• Must work with FOS User Bundle

Nice to haves / consider:

• It should be possible to make the invitation code optional for apps we build for the general public

Existing implementations:

• FOS User Bundle has a doc on adding invitations. This approach only allows a one to one mapping of user invitations.
• Costimator: We implemented this in Costimator but it involved overriding the RegistrationController and SecurityController. I looked at using the FOS Events but they don't allow us to add additional processing of the form.

Ping @iainsaxon @ianbelcher

[leevigraham]

Pretty sure this will break the Costimator implementation as we add an error when we create the form if the invitation is not valid (before the form is submitted)

https://github.com/symfony/symfony/commit/9dfebd529edde1f65664a37b009179a78d496335

[leevigraham]

Here's the user invite flow for ClubEvo.

The first invitation code check could be done with a listener that exists early. This would mean the user would not even see the login / register which would be bad. I think catching this in the controller would be better and displaying a flash message.

Not sure how we would handle re-authenticating

[leevigraham]

Just had a thought. Maybe all invitations should link to /invitaions/claim/xxx. That way any checks can be made to see if the invoice is valid, if the user is already logged in etc. Basically a request proxy to determine the response.