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.
What's the reason for this change?