Summary:
At some point, the highlighting behavior for the timeline broke. When you follow a link to a particular timeline story, the story should be highlighted.

Prior to this change, the `<a />` tag itself highlights, but there's no associated CSS and it's too deep in the tree to do anything useful.

(Since this change is fairly straightforward, I gave up digging for the root cause before finding it.)

Test Plan:
  - Clicked a timeline story anchor, saw the story highlight.

Differential Revision: https://secure.phabricator.com/D21213