New /feature landing pages

This set of changes are around creating new /feature/<ID> landing pages for each individual feature.

We'd like each page to be indexable and serve as the de-facto landing page for each feature, so the feature's metadata is displayed in plain HTML (not a Polymer element), and the site's Atom feed uses the corresponding landing page as the <link rel="alternate"> for each item.
Ah, I was wondering. It looks like [the bug](https://bugs.webkit.org/show_bug.cgi?id=110786) was fixed a year and a half ago in WebKit, so presumable it's okay now. I haven't noticed any performance issues.