Set an explicit ordering on Videos

Django throws a warning if you paginate a queryset without an explicit
ordering.

wagtailvideos/models.py

@@ -280,6 +280,7 @@ class AbstractVideo(CollectionMember, index.Indexed, models.Model):
 
     class Meta:
         abstract = True
+        ordering = ['-created_at']
 
 
 class Video(AbstractVideo):