Invitation should have a reference to the invited user

[magick93]

It would be good if the basejump.invitations had a reference to the user who has been invited.

There are two reasons why this could be useful:

1. With a constraint, it can prevent spamming of the user to join the same group
2. As an account owner, it is then possible to see a list of users who have been invited. And, as an account owner, if I cannot see who I've invited, I'm likely to invite the again.