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.
Was needed because this change introduces div elements (in replace of tables) for doing the layout.  That obviously screwed up this test because the CSS selectors are now longer specific enough.

I'll look at changing the test to use something other than divs so as to avoid the clash and get the test back to something equivalent to what it was.