Save And Return

* reinstate save and return page at start with memorable word
* Email form number to recipient
* return to saved form page

Added a databaseless model (PlainModel) so non-database models can be treated the same as normal models (e.g. validation, saving, etc).

May want to change the path (user_session) to something a bit more user friendly.

Errors are on the specific fields but appreciate we may not want this but there is no styling setup for base errors. Plus memorable word will need changing/removing/copy editing once we speak to IA so this can be revisited then.

Went with mocking in user_session specs as I prefer to isolate tests and cover integration with a feature spec. Let me know if we're using another strategy.