Add ability to re-send account creation emails.

[teymour-aldridge]

Submitting the "invite user" form a second time will now send a new email.

Fixes #2582

[teymour-aldridge]

For

     if created:
        user.membership_set.create(group=self.cleaned_data['role'])

This might cause some confusion if a user is invited once with some role, but the follow-up email is sent with a different role (as the second role will be ignored). However, I think this is more reasonable than changing the user's role here (this should be done separately if necessary)?

[tienne-B]

@teymour-aldridge That is a good point! A user can have many groups, and different groups between tournaments, so that new user check is problematic.

Have you tried using the .add() method instead of .create()?

I think it'd look something like this:

user.membership_set.add(self.cleaned_data['role'])