plugin-ify the non-dag code here

[7yl4r]

I stumbled today across the airflow plugins docs and realized that the dag_classes and operators here could (should?) be defined in a plugin.

It might be nice to separate all that code from the dags themselves so this repo is less overwhelming at first.
There is other functionality offered by setting up as a plugin too.

So that's the right way to do it, but what we have is working for now so I guess this goes waaay down the queue.