rest: standardize on field_id

[sheppard]

Foreign keys are serialized as (field)_id, but when saving a model, the field name must be sent without the id suffix. Among other things, this makes form rendering a bit awkward (<input name=field value={{field_id}}>). The best approach is probably to standardize on field_id, and figure out how to remove the field_from_native / field_to_native hacks in IDRelatedField.