these were named as BookwyrmGroup and BookwyrmGroupMember due to a misunderstanding about related_name and a dodgy development environment. This naming makes more sense.
- notify group members when a new member accepts an invitation
- notify all group members when a member leaves or is removed
- notify ex-member when they are removed
- add additional logic to visible_to_user, for groups and their objects
- cleans up some queries in Group view
NOTE: I can't work out how to make group lists only visible
to users who should be able to see them, on user group listings.
They still can't access the actual group, but can see it on
user pages. This is potentialy problematic.
- display groups on user pages when not the logged in user
- restrict visibility of groups on user pages and group pages themselves according to privacy settings
