This adds to new fields to the BaseModelView: `allow_frozen_fields` and
`excluded_frozen_fields`. Setting `allow_frozen_fields` to True adds
another button to the create and the edit view which acts like
"Save and Add Another", but "freezes" the fields (i.e.,
doesn't reset their contents).

`excluded_frozen_fields` can be used to specify fields
which should always be cleared (e.g., password fields).