Fix preview of connected stories.

The "Preview" link in the story builder was linking to the normal story
viewer for connected stories.  This is problematic first because the
section titles aren't hidden and second because users will never view
a connected story in the story viewer.  Instead, they show up in the viewer
of the seed story.

To fix this, I had to change a few things on the back end.  I needed to
include draft connected stories in the template context for story viewer.
I did this by overriding the ``get_context_data`` method of
``StoryViewerView`` and passing a ``connected_stories`` context variable
that contains the filtered connected stories list instead of calling the
story instances ``connected_stories`` method from the template.

I updated ``Story.connected_stories`` to take arguments that would cause
it to return either only published connected stories or published connected
stories and drafts for the logged-in user.

To activate the different connected stories behavior, I added a
``preview`` url route that passes a ``preview`` keyword argument to
``StoryBuilderView``.

The preview URL is then passed to the Backbone views by ``StoryBuilderView``
as a template string (so the builder can parameterize it with the story
ID to handle the case when a new story is initally saved).

Finally, I updated the story viewer Backbone views to properly display a
connected story when accessed directly by it's hash-based URL in the
browser's location bar (rather than being accesed through clicking links
in the view).

Addresses #601.
Use jQuery to detect element width

In ``ViewerApp.sizeFigCaption``, ``this.width`` was being used to detect
if the element had a width.  This was causing the caption and the bounding
element to be incorrectly sized (or even have the size set to 0) in
``ViewerApp.sizeFigCaptions``.  Checking the width with jQuery's ``width``
method seems to make things work correctly.  The ``width`` method returns
the current computed width of the element, which is what we want rather
than the element's raw width value.

Addresses #601
Remove ununsed template tag

I think this was an artifact of the old AddThis sharing markup.
Properly populate preview bootstrap section list

When there were no published connected stories, the sections Backbone
collection wasn't getting bootstrapped with the draft connected stories
when in preview mode.  By passing an optional iterable of connected stories
to the ``BaseStructure.stories_json`` method, the draft connected stories
can trigger the inclusion of the connected stories list in the viewer
navigation when previewing a story. The logic for the getting the correct
connected stories iterable is done in ``StoryViewerView.get_context_data``.

Addresses #601
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
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.