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.

Looks good. One of the only things I would change would be to make this a helper method. This would obviate the need to explicitly call `build_user_session`, e.g.

```ruby
helper def user_session
  @user_session ||= UserSession.new
end
```

You'd probably need to add `PlainModel#update` so in the create method you could do this:

```ruby
def create
  if user_session.update user_session_params
    session[:claim_reference] = user_session.reference
      redirect_to page_claim_path(page: 'claimant')
    else
      render 'new'
    end
  end
end
```

This would allow us to get rid of the nil guard in `user_session_params` too.