Django further model registration via signals

The use case would be do something/register models that are a subclass
of one of the classes defined in this models.py file - e.g.: to
automatically setup a monitoring sensor, or similar), without the need
for each such plugin application repeatably calling get_apps

* This is a proof of concept code
* I'm not even sure if this is the Django way
* I'm not even sure it's useful and get_apps would be a real
  performance hit, and explicit calling of it wouldn't be clearer