Integration with Django's system check framework

[wesleykendall]

Integrate pgtrigger into django's check system similar to other Django model constructs. Key pieces of behavior desired:

1. Duplicate trigger name checks happen in the check system instead of during code load time.
2. Warn users when child models don't inherit the triggers of the base model

Although Django allows one to turn off checks, number 1 will still fail during installation time since a duplicate trigger name is a serious error.

Number 2 should be able to be ignored if it happens, although it's assumed this should be a rare case. It is not totally clear if/how number 2 can fit into Django's check framework, so more research is needed.