disambiguate groups and prep for group invitations

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

bookwyrm/forms.py

@@ -297,7 +297,7 @@ class ListForm(CustomForm):
 
 class GroupForm(CustomForm):
     class Meta:
-        model = models.Group
+        model = models.BookwyrmGroup
         fields = ["user", "privacy", "name", "description"]
 
 class ReportForm(CustomForm):