Replace approrpraite __in with many-to-many relation query

[klnusbaum]

There are several queries that involve selecting a QuerySet using the __in selector where the argument is a set of primary keys. We should be able to replace those with a more appropriate and probably more efficient many-to-many relation query as described here: http://stackoverflow.com/questions/16431151/django-select-users-from-seperate-model