Fix bug 1077879 - Add an HTML view

*This builds on PR #18 - you may want to merge that first*

These changes add HTML views of features and related data.  This code has been applied to the Heroku instances.

The HTML variant of the API view_feature constructs the HTML on the server side:

https://browsercompat.herokuapp.com/api/v1/view_features/741.html

The JS-enabled view downloads the JSON API representation, constructs the HTML version, and injects it into the DOM:

https://browsercompat.herokuapp.com/view_feature/741

This change makes it easier to visually confirm that an MDN page has been imported correctly:

https://developer.mozilla.org/en-US/docs/Web/CSS/float#Specifications