Don't show connected stories in "Latest Stories"

In order to implement this, I changed storybase.utils.latest_context to
take a Queryset as the first argument instead of a Model class. This lets
me do more filtering on the per-model side.

I also updated storybase.utils.latest_context to sort the lists by the
last_edited field instead of the published field.  This makes it consistent
with the sorting on the Explore page and on the Project and Organization
list views.

Addresses #609.
Hide connected story viewer and detail views

Detail and viewer views don't make sense for connected stories because
they're only meant to be shown in the story viewer of their "seed" story.
So, accessing their detail and viewer views should respond with a 404 error.

I also updated the "My Stories" view so that the title of connected stories
no longer links to the detail page, but shows an icon to help identify it
as connected and links to the seed story's detail page.