Refactor patch_session() into more something more generally useful

[RobRuana]

patch_session() looks like a code smell, so we should eventually refactor it. However, I'm marking this issue as low-priority because MAGFest won't derive any immediate value from fixing it.

[EliAndrewC]

The implementation is indeed wonky; I doubt that monkeypatching the session instance is anywhere close to the best way to provide that kind of functionality.

I was basically trying to do stuff like this:
https://docs.djangoproject.com/en/1.10/topics/db/managers/#adding-extra-manager-methods

At the time I wasn't able to figure out a more supported way of doing it. Re-reading through the docs a bit, I think the correct approach is to subclass Session and then pass that as the class_ parameter to sessionmaker:
http://docs.sqlalchemy.org/en/latest/orm/session_api.html#sqlalchemy.orm.session.sessionmaker.__init__