these were named as BookwyrmGroup and BookwyrmGroupMember due to a misunderstanding about related_name and a dodgy development environment. This naming makes more sense.
Let's do this the sensible way huh, by using backwards references to memberships etc
Also adds filters for is_member and is_invited so we don't have to do weird things in group Views
- search for users to add to a group
- display suggested users on search results screen
TODO: actaully enable users to be added!
TODO: groups/suggested_users probably could be replaced with some logic in snippets/suggested_users.html
