Don't try to create notifications for authorless stories

While this case shouldn't occur in actual system use, in tests, the
``create_story`` API function is often called without an author specified.

The signal handlers used to create ``StoryNotification`` model instances
when a story is saved were failing when trying to check the
notification settings in the profile for the nonexistant story author.

This commit adds a guard statement to check that the story has an
author before proceeding with creating a notification.

Addresses #580
Pass include_host when testing method

As part of implementing #580, the API for
``Story.featured_asset_thumbnail_url`` changed so that the hostname
is included in the returned URL by default.  This breaks the
``storybase_story.tests.StoryModelTest.test_get_featured_asset_thumbnail_url_empty``
test.  Passing ``include_host=False`` to the call to
``featured_asset_thumbnail_url`` matches the old behavior that the test
is expecting.