RSS feed for stories

Add a new ``django.contrib.syndication.views.Feed`` subclass and
URL configuration for an RSS feed of recent stories and one for
recent stories filtered by Topic.

To try to correctly get the ``length`` attribute of the RSS item
enclosure, in our case the featured image file, expose the
featured asset's ``Thumbnail`` object (which, like all
Django file-like objects has a ``size`` attribute) via a new
method called ``LocalImageAsset.get_thumbnail``. Refactor
``LocalImageAsset.get_thumbnail_url`` to call this method.

Addresses #816