Create "core" app with utilities and other shared code

[hdoupe]

There are several functions that are duplicated in the dynamic and taxbrain apps. Further, several variables, functions, and classes in taxbrain are imported into dynamic. I think we should move this code into a utilities app named something like "core" or "utils."

Once we've accomplished this first part, we should begin looking at code in the views.py and models.py modules and thinking about how we reduce the amount of redundant code.

cc @codekansas

[hdoupe]

cc @kpinkelman