allow Invitation key to be passed to the redirected url

[shawnzam]

Allow the Invitation key to passed to the redirected for further inspection. This is used to implement a pairing model in my case.

[valberg]

I'm not entirely sure how django behaves when providing a keyword argument to an URL which does not take said argument. We would have to make sure that this is backwards compatible.

[blag]

Judging from here and here, you should be able to pair inviter/invitees before the redirect is returned by defining a signal handler that connects to invitations.signals.invite_accepted:

@receiver(invite_accepted, sender=Invitation)
def handle_invite_accepted(sender, email, request, invitation):
    inviter = invitation.inviter
    invitee = request.user
    ...

@shawnzam Does this work for you?

[Flimm]

Thanks for this contribution. I'm not sure if other people will find it helpful, especially given the workaround given by @blag. I'm closing this pull request to avoid it stagnating.