Python: Make django views/fields/forms class modeling extensible

This also requires that we make this part of the modeling public, which I guess
is step we want to take eventually anyway!

I'm not quite sure whether the modules `Django::Views` and `Django::Forms` are
actually helpful, or whether we should just have their modules available as
`Django::View`, `Django::Form`, and `Django::Field`...

Author: RasmusWL