Improve group data model

[signebedi]

It may be worth adding a last_edited timestamp field, and a last_edited_by foreign key > User.id field. This enables slightly better audit trail. Cf. all related transactions will be stored in the transaction log.

[signebedi]

However, deleting a group causes the group to be removed from users. I think maybe a better way handle deletion is to maybe add an is_deleted bool field, or to simply strip all permissions from a group on-delete. Or, both. Then, if a user is a member of a group that is deleted, it will show up on their profile, but we can add a (deleted) designation to show the group is inactive.