Massive cleanup of CSS and related code

This was long overdue, and will help as we refactor, simplify, and add features. Already seen a couple of long-standing UI bugs (e.g. the scratchpad being offset upwards) that were fixed in the process of this cleanse.

* Removed the vast majority of `khan-site.css`, most of which didn't apply at all to our codebase.
* Removed the jQuery UI code from `khan-site.css`, and added the external jQuery UI library CSS to pages that need it.
* Removed some classes and divs from templates, as they weren't being used.
* Cleaned up some formatting (mostly indentation).

I used tools to scrape the site and determine which CSS rules weren't being used, and then went through manually and removed/tweaked the rules in the CSS, with discretion, testing as I went.