New general layout using <div>s

Follow up to https://github.com/jenkinsci/jenkins/pull/1286.

Hoping we can move with this one.  One issue identified by Daniel Beck was how the main panel is not filled all the way to the RHS.  I'm fairly sure (still need to play some more with it) that's because of the 100% tables, but would like to work on that separately if possible.
The `div` here means "search for CSS classes named `.searchbox`, but only on `div` elements" and should lead to unnecessary searching, especially because I'm pretty sure there's only one page element with the `.searchbox` class, and probably not one that's a `div` and another that's an `a`. If the second case is true, better to just add another CSS class for the div element and style that.

For more see http://css-tricks.com/efficiently-rendering-css/, it's a little out of date but covers the main points.