Make it simpler to instantiate subcommittee-like Organizations

[mileswwatkins]

A to-do from the Open States documentation:

If you're instantiating a subcommittee, you'll need to pass ``parent_id``. ``parent_id`` can be:

    * another instance of ``Organization``
    * a dictionary, like ``{'name': 'Appropriations', 'classification': 'lower'}`` which will find the House Appropriations committee at import-time

    .. TODO: ^this is sort of a weird edge case, and could probably be handled a lot better in pupa

cc @jamesturk

[fgregg]

There's a couple of other places where it's currently necessary to use this dictionary approach.

Besides subcommittee:

• bill actions
• related_entities of a bill action