Remco usergroups, Django 1.7

Move to Django 1.7; add UserGroups.

Unlike the earlier pull request, this one will not be released tomorrow, but only when full administration pages for user management are done. There may still be Django 1.7 issues left, but right now the tests and migrations work and we will find other issues during development of the admin pages.

As there is only a single site with this app, it's OK to just throw out the old South migrations, making this a decent first site to move to 1.7.

UserGroups are the feature added: users can be added to usergroups, usergroups can have roles, the users in the groups automatically have those roles too. This will be the main way of managing user roles in the upcoming administration pages.