optional additional params, when sent to /:event_id/newuser, automatically reidrect to Driver#new or Rider#new

[AaronHarris]

Basically, /19039094384/newdriver => events/19039094384/drivers/new
and
/19039094384/newrider => events/19039094384/riders/new

Not sure whether we want it to be /19039094384?type=driver (or events/19039094384/newuser?type=driver) or /19039094384#driver or simply /19039094384/newdriver

In future version of app, where driver = user + car, it will route to the new user page with the option to drive ('I have a car') already selected.