Allow changing of language preference from profile page

This adds profile API functions for handling the fetching and updating of user preferences, a language selection form to the profile page, new JS to process the form's submission, and modifies the profile views to handle this new functionality. Also includes a handful of small tweaks to make things more consistent.

@wedaly and @stephensanchez, could you review this?
This code seems out of place in `student_profile/views.py` since it includes business logic about language preferences.  I'd prefer to move this into an `api.py` module (which you'd need to create) in `lang_pref`