Modify login and registration panels per wireframes

Both:
* Loop to render fields instead of using `form.as_p`
* Remove JS code to add classes to the template output
* Add ":" after labels using CSS
* Further styling to be added by Design team

Login page:
* Subclass `AuthenticationForm` so we can add CSS classes server-side

Registration page
* Place fields side-by-side by adding `outer_class` to field widgets in `RegistrationForm` and digging down to retrieve them in the template
* Insert "Additional Information" header
* Preliminary SASS for styling