Adds opt-in field.

# Changes
 - Adds opt-in checkbox for transactional email.
 - Adds nicer checkbox styling, courtesy of [WTF, Forms](http://wtfforms.com).
 - Adjusts drawer close button to avoid layout issues on tablet and mobile.
 - Some refactoring in the sessions controller.

For review: @angaither 
the only thing here about `Input::all()` and calling `new User` I would be cautious about, is to make sure that the new users are getting set with null email or phone number depending on which form they are given