Add current page notifier and improve contents box

[samuelsidler]

In the handbooks, there's currently a few visual annoyances:

• No indication in the sidebar of which page you're on.
• "Contents" box (generated from headings) isn't a box at all and is just inline.

I've made some improvements to this in the polyglots handbook, but haven't checked them in yet. I'd like feedback and help on the following things before checking in the changes:

1. The CSS work on the contents box still isn't right. The content area should wrap around it. ​On this page, the header doesn't wrap properly. ​On this page, the yellow "draft" box doesn't wrap properly.
2. Green probably isn't the right color for current page. Right now, we use red on hover across WordPress.org. I picked green, but any color would work. Ideas for the right color? WordPress orange? Would WordPress orange clash too much with the red on hover?

Any help or feedback would be appreciated.

[samuelsidler]

In the attachment above, @sonjanyc worked up a design that we can use to improve a bunch of things. I can implement most of them in CSS, but not all.

We still need code changes to fix #439, #437, and #436.

We may also need some further design changes/fixes. Patches always welcome. :)

[samuelsidler]

In 550:

P2 theme, handbook pages: Add new design to handbooks. Fixes #430. Props sonjanyc