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?
You can just do:
```
profile_api.update_preference(self.USERNAME, preference_key='preference_value')
```