Section relationship refactoring

At the moment section membership relationship is identical to group membership relationship, so sections are being listed in the groups list of a person, while both groups and persons are listed in the members of a section.

This requires constant filtering by object type in all the places that are accessing section.members or person.groups.

It should be fixed by creating new section-member and section-group relationships, evolving old databases to use these new relationships and modifying old code to use the right attributes to access data instead of filtering.