Fix display of connected story byline

A typo in the template was causing connected stories to show the seed
story's byline instead of its own.

Closes #607
Merge branch 'hotfix-607-connected_story_bylines' into develop
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.
Merge branch 'hotfix-609-connected_in_recent' into develop
Forefront connected stories in latest stories list

Connected stories shouldn't be shown in the latest stories list, but they
should be forefronted somehow.  The way I've decided to do this is to
use a signal handler to update the last_edited field of a Story model when
a connected story is published. That way, stories with new connected stories
will be bumped to the top of the "Latest Stories" list on the homepage.

I also added a small link and icon reporting the number of connected
stories that links to the connected stories section of the detail page.

In order to do this, I added extra context to the the dictionary returned
by ``Story.normalize_for_view`` and updated the ``latest_stories`` template
tag to use the ``storybase_story/latest_stories.html`` template instead
of ``storybase/latest_objects.html``.  I added an ``extra`` block to the
``storybase/latest_objects.html`` template so templates that wanted to add
extra stuff could just inherit from ``storybase/latest_objects.html``.

Finally, I wired in a Google Analytics event to track how many time users
click this link to see if it's worthwile to take up space.

Closes #609
Merge branch 'hotfix-609-connected_in_recent' into develop
Merge branch 'develop' into 515-publish_cleanup