Fix html validation issues (and other stuff)

I fixed a bunch of HTML5 validation errors (#5):

- For some reason, the W3C validator complains about the self-closing tag `<i class="icon-ok"/>`, but we write it as `<i class="icon-ok"></i>`. I tried to fix it by inserting an `&nbsp`.
- We need to escape `&` in URLs, so I just run the entire S3 URL through an HTML escaper.
- Changed a `<span>` to a `<div>`, since we can't nest `<div>`s inside `<span>`s.
- `<th>` tag can't use the width attribute, so I switched it to using CSS

I also moved the internal stylesheet into an external stylesheet. I believe this should improve load times, since we download the CSS only once, instead of for every page.

Finally, I upgraded to jQuery 2 (#4). It's smaller (and reduces load times) because it drops IE 6/7/8 support. However, I decided to keep jQuery 1.9 around with conditional comments. We still support IE 6/7/8, but we should also get the speed improvement, since jQuery 1.9 is only downloaded by the browsers that need it.

Reviewers: @cpkleynhans