My memberships feed should return groups sorted on their lastModified date

 - It's mostly an implementation detail, but when you create a group it doesn't have an old `lastModified` timestamp so there's no need to update anything in the libraries.
 - The leave event doesn't have an activity. If somebody left the group, the group would be bumped to the top but it wouldn't be apparent as to why as there would be nothing in the activity feed.