Remove unused import
Add not_published method

Add a ``not_published`` method to ``FeaturedQuerySet`` and
``FeaturedManager`` and use it in ``storybase.utils.local_context``
Remove unused imports
Remove unused imports
Use custom query set methods

Where possible use ``FeaturedQuerySet`` and ``PublishedQuerySet``
convenience methods to improve code brevity and clarity.

There are a few places where this isn't possible, such as when the
publication status of an item has to be checked at the same time as other
fields by "|"ing two Q objects together, but in general these changes
provide cleaner code and much better hiding of implementation details.