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.
![image](https://cloud.githubusercontent.com/assets/1749548/3809544/6cadfb08-1c85-11e4-9590-edc4dfa2369e.png)