disambiguate groups and prep for group invitations

- rename Group to BookwyrmGroup
- create group memberships and invitations
- adjust all model name references accordingly

bookwyrm/models/user.py

@@ -143,6 +143,11 @@ class User(OrderedCollectionPageMixin, AbstractUser):
     property_fields = [("following_link", "following")]
     field_tracker = FieldTracker(fields=["name", "avatar"])
 
+    # @property
+    # def bookwyrm_groups(self):
+    #     group_ids = bookwyrm_group_membership.values_list("user", flat=True)
+    #     return BookwyrmGroup.objects.in_bulk(group_ids).values()
+
     @property
     def confirmation_link(self):
         """helper for generating confirmation links"""