Use CELERYBEAT_SCHEDULE to schedule tasks

This is the prefered way from celery because it's more flexible.  Most
importantly for us though, doing it this way prevents us from creating
duplicate tasks when we have duplicate modules (for example
apps.videos.tasks and videos.tasks)
Fixes for the video feeds tests
Speedups for updating video feeds

Check for existing items using a single query instead of calling
Video.get_or_create_for_url for each item.  This reduces the DB load
and also avoids running the
_add_amara_description_credit_to_youtube_vurl task.