Use field.choices in form_extra.py to create dropdowns for fields
Loosen up identifier regex, allowing variables not named after models

Whitelisting the model part of model.field was a bit of a misnomer, since
it whitelisted the template variable name, not the actual model class it
represented.

Identifiers still need to be of the form "model.field" though, in order to
access the field meta information on the model class