refactor GameObject and ScriptedObjectMixin

[vilmibm]

For readability I cut a bunch of WITCH-related code into scripting.py and defined a mixin used in models.py. this creates some unfortunate circular dependencies and has ended up being more confusing than it's worth.

I'd like to try and keep separating the witch scripting stuff from the database stuff, but they're very entwined.

I think at the very least splitting models out into a models/ directory and giving each class its own file will help.