Use Backbone for student account and profile JS.

Validate student account and profile form fields. Use RequireJS for Jasmine tests of account and profile JS.

@wedaly and @dianakhuang, could you review this?
Agree with merging it and refactoring in a future PR.  We're going to need to make  `/profile/preferences` accept GET requests soon anyway (in order to choose the user's language in the menu on page load), so we can refactor both views then.