In the previous setup the GroupsFinder class had two distinct tasks:

1. Finding the projects user A could see
2. Finding the projects of user A that user B could see

Task two was actually handled outside of the GroupsFinder (in the
UsersController) by restricting the returned list of groups to those the
viewed user was a member of. Moving all this logic into a single finder
proved to be far too complex and confusing, hence there are now two
finders:

* GroupsFinder: for finding groups a user can see
* JoinedGroupsFinder: for finding groups that user A is a member of,
  restricted to either public groups or groups user B can also see.