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 @dianakhuang, could you review this?
I don't think we do this in the code as it is now, but I think it might make sense to do some validation on the new language to make sure it is a valid one.