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
Use topic-exclude instead of topic-ne

Use ``topic-exclude`` as the querystring parameter for excluding
a topic from the story feed instead of ``topic-ne`` as it seems much
more human-readable and easy to remember.

Addresses #816
Correctly set the MIME type for story feed

``guess_type()`` returns a tuple.  Only return the first item
(the type).  Returning the tuple was causing the type to be shown
as "('image/jpeg', None)" in the feed and was causing validation
errors.

Addresses #816
Exclude connected stories from feed

Addresses #816
Truncate story summaries in feed

Truncate story summary to 75 words, which is the same length as the
featured items on the homepage, in feeds.  This was needed because
long summaries didn't work well when pulling the feed in for
MailChimp integration.

Addresses #816
Merge branch '816-story_feed' into develop
Update changelog for #816