Resolves reuse of invitation code

Closes #1952 (closed). See comment

With this MR the invitations are personal and cannot be used twice, so once the invited user is deleted the invitation is also deleted. I think this is the solution that makes more sense. I also coded a version where the invitation code was deleted after being used but it was too awkward: it just doesn't feel right to have anonymous invitations.

Now the administrator can see who has been invited.

Before:

Note:

manage.py migrate must be run