Start refactoring dynamic UI.

1. When using $value for form field attribute, first try
cleaned_data. And only if it is absent, use raw data.

2. Move all service fields classes into separate module fields.py

3. Move some helpers to helpers.py and rename __common.py to forms.py.

Change-Id: Iba27cb2f9bc1c017443e0666aa274b5f88373045