Nav highlighting is fragile and breaks easily

[nickbristow]

No description provided.

[mbland]

I've been making this more robust in several 18F/guides-style PRs:

• update_nav collection, default permalink support guides-style#26
• Build nav based on permalinks, not parent titles guides-style#28
• Fix child nav expansion broken in #28 guides-style#29
• Remove new images.png, recursively remove child nav items guides-style#30
• Support short nav titles, expanded nav bar guides-style#32

The upshot being, ./go update_nav will be a pretty robust, functional means of keeping the nav bar in sync with the pages once the last of these PRs makes it in.