Remove unused import
Update template names in StoryWidgetViewTest

Update tests in StoryWidgetViewTest test case to reflect the new
template names that were a result of the changes in
eeda3a1201ef97b61a520faab495260b30df36c8.

Addresses #741
Add widget_content block back to widget_base.html

The ``widget_content`` block was removed from the
``storybase_story/widget_base.html`` template in
c0f3cf29723b95c1ffc8463efe10460528a4926c, which broke the
inheritance in ``storybase_story/widget_404.html``.

Addresses #741