Simplify registration process for users
This MR tries to simplify the user registration process by pre-setting bundle and/or billing period depending on the URL used for registration.
More detailed changes:
- Move cart's form from carts/views.py to carts/forms.py.
- Add url field to ChosenReward model.
- Use new ChosenReward url field in profile view.
- Add new configuration option WOO_REG_OPTIONS_INFO_FILE that specifies the registration options available in the middleware.
- Remove not used WOO_CART_xxx configuration options.
- Define new registration URLs in middleware/urls.py.
- Add example configuration file registration_options.yml.
See #86 for more details.