redirect users back to most recently viewed page after login

user story https://www.pivotaltracker.com/story/show/64997344
based on recommendations at the devise wiki
https://github.com/plataformatec/devise/wiki/How-To:-Redirect-back-to-current-page-after-sign-in,-sign-out,-sign-up,-update
with slight modifications to request paths that are ignored in the store location method.
rather than hardcoding the paths, use the route helpers:

~~~ruby
[ new_user_session_path, new_user_password_path, ... ]
~~~