Builder: form validation and module name as prefix

- Form validation was accidentally removed from the generated
controller. Re-added it using the model's validation rules.
- Call model's prep_data() method to handle input, still sets some
default values in the controller if post data for date or datetime
fields is empty
- Fix one place where module name was being added as the field prefix
whether the option was enabled or not (though the option itself still
needs to be added back into the view)