replace Django forms.* objects with names of forms.* objects

Remove Django-specific code so it's possible to use the model
*_FIELDS in non-Django apps.
refactor ddrlocal.forms to one form into which you pass *_FIELDS

Previously, there were basically identical forms for each model
(collection, entity, file), and the *_FIELDS data structures
were imported.
With this change there is now only a single DDRForm.
*_FIELDS dict is passed to DDRForm object when instantiating.
The idea is to make it possible to move *_FIELDS into webui or
some other app.
remove Django settings object from *_FIELDS modules

The ultimate idea is to make *_FIELDS useable outside the context
of Django, e.g. by a Flask app or a command-line tool.
sample MODEL_FIELDS data structure with a single field with notes