Add featured story count

Add the featured story count to the links at the end of a story that is
featured in the homepage slider.

Addresses #629
Show link to connected stories in explorer

Show a link to the connected stories in the explorer.  In order to do
this, I added a read-only ``connected_count`` field to the StoryApi
response. This is generated by the ``Story.connected_count`` utility
method.

As the number of stories with connected stories is somewhat small, there
doesn't seem to be any noticeable performance penalty for the extra
database call to get the number of connected stories. If this changes,
we could cache the value and invalidate using a signal, similar to how
other ``ManyToMany`` fields on the story model are handled.

Closes #629