Reintroduce highlighting for permalinks.

Highlights and scrolls to the appropriate post if a post ID is
in the anchor.

I've jiggered around with the formatting a lot, so here's a screenshot
from before the change:
https://www.dropbox.com/s/mqx16cz0clrsd9a/Screenshot%202014-06-24%2012.58.24.png

And here are two screenshots from after:
https://www.dropbox.com/s/88oqolhl28vxngo/Screenshot%202014-06-24%2012.56.23.png
https://www.dropbox.com/s/drbwq3gpykyeva9/Screenshot%202014-06-24%2012.59.15.png

By the time honoured technique of flipping rapidly between tabs,
I checked that the layout does not change visibly except for the
addition of the green bar.

Still needs the ability to link to a post below the fold
(but this is not a regression from master).

Change-Id: Ifde2bd3b5d4afb1e13766edb55980d2c3949e339
Merge "Reintroduce highlighting for permalinks." into frontend-rewrite