Fix the permissions on adding communities

When we add a community in the django admin interface, we also make sure there is an associated group. This group needs to provide permissions so that stuff users can edit entries in the admin interface.

The old way to filter for the permissions missed some entries on our staging vm. The reason is still unclear for me but it seems that those permissions can get out of sync with the database models.

The new approach fixed that, in a way that it doesn't filter directly by the user.

Closes #32 (closed)