replace Modernizr with html5shiv

The only thing we're using Modernizr for is to insert html5shiv.js so that some elements display in non-quirky ways on IE8 and IE7.

By replacing Modernizr with html5shiv, we reduce the JS footprint in IE8 and IE7 and can eliminate it entirely in other browser thanks to conditional comments. (I didn't bother eliminating it in places where the script tag was being injected by PHP code--this is still a performance improvement over downloading and executing Modernizr--but someone may take the time to do so if they wish. I suspect what is more likely to happen is that at some point, IE8 won't be relevant anymore and we can just take out html5shiv entirely.)