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.
This was collapsed because whitespace nodes made `<template>` instantiation very slow. I believe this has been fixed in Blink, but it's something to watch out for if we notice a perf hit.