Add comment to widget.less

"Widget" is such an ambiguous name, so add a comment to explain
the purpose of the stylesheet, how to compile it, and the related
template.

Addresses #741
Refactor widget templates

I prefer a template scheme that leverages inheritance instead of
using ``{% include %}``. So, put the content in
``storybase_story/widget_list_content.html`` and
``storybase_story/widget_story_content.html`` in
``storybase_story/widget_base.html`` wrapped in ``{% block %}`` tags.

Then in ``storybase_story/widget_story.html`` and
``storybase_story/widget_storylist.html``, override the unneeded blocks
with empty content.

``storybase_story/widget_storylist.html`` becomes an empty template, but
I decided to keep it to allow for future customization.

Add better docstrings for ``StoryWidgetView`` and template files.

Addresses #741
Fix viewer URLs in widget template

Wrap URL name in quotes to match new-style syntax.

Addresses #741