Initial pass at user login/registration modals.

Notes:

 - Form submission causes an issue on localdev (posts to `https://...:8888` rather than `https://...:8889`) :warning: See DoSomething/dosomething#613.
- Login from `user/login` or `user/registration` still works normally.
 - May have to be re-implemented if we end up writing our own user login module to replace the default login/registration forms? See DoSomething/dosomething#614.