More than one private invite per user causes InviteView to throw an error

[brookiel]

Adding more than one private invitation causes InviteView to throw an error.

valid_code = InvitationCode.objects.get(owner=self.request.user,
                                            num_invites__gt=0)

Unless we are tracking the number of invites left elsewhere and creating codes only when needed, I can put in a pull request to fix this tomorrow. Maybe a template for out of invitations is in order.