Handle widget versions in the same view

The likely part of the widget to change between versions is the
template, so it doesn't make sense to have separate view classes.

Instead, just implement the ``get_template_names()`` method to
also search for a version-specific widget template.

Addresses #741
Views that include lists of stories in widgets

Checkpoint commit for implementing views that list stories for
a particular taxonomy term and include stories for a particular
term along with a featured story.

Addresses #741