Display names, help text, and "deprecated" for Advanced Settings.

Hackathon with @frrrances and @mhoeber 

@dmitchell Please review the Python/JS code. Much of it should be familiar to you. :) Mark will be updating the user-visible Strings, but all functionality is working.

Depends on https://github.com/edx/XBlock/pull/223 (which has already been merged).
Why the abbreviation? I'd much prefer modelValue...