Bootstrap 3.0.0-rc1 upgrade

*So, I thought I'd opened this yesterday morning, but apparently I made a pull request (and added comments) to myself. Weird.*

I took this upgrade as a chance to do some clean-up and make some changes in how we're doing some things, so I'd welcome any feedback.

One major change is I switched all the modal dialogs (scratch Activecode, compare me, codelens "check your understanding") to use the modal classes that Bootstrap provides, instead of using the jQuery SimpleModal dialog. I think that the BS modals have better visual integration, and they're a little easier to style. They are light colored by default, but it shouldn't be terribly difficult to emulate the darker background that SimpleModal used by default if you think that looks better.

I changed the icon for the social/follow menu in the navbar. I used another icon from the Glyphicons set, so it better matches visually with the other icons. I picked one of the ones they call a "Share" icon, although I'm not completely convinced it's the right one. Take a look at http://glyphicons.getbootstrap.com/ for some other options. The thumbs-up one might be a good possibility. (I like the Glyphicons because they're really well integrated into Bootstrap, and this particular set is license-free for Bootstrap users but I'm open to finding other icons).

I'd say that this is ready to merge in. I'm sure we'll find other things to fix in the next few days.