Fix filtering for already-posted news articles

We need to filter based on just the keys, not the values as well, when
figuring out if there is anywhere left to post an article.

Author: mhagander

pgweb/news/management/commands/social_post.py

@@ -33,7 +33,7 @@ def handle(self, *args, **options):
         if not curs.fetchall()[0][0]:
             raise CommandError("Failed to get advisory lock, existing social_post process stuck?")
 
-        articles = list(NewsArticle.objects.filter(modstate=ModerationState.APPROVED, date__gt=datetime.now() - timedelta(days=7)).exclude(postedto__contains=allprovidernames).order_by('date'))
+        articles = list(NewsArticle.objects.filter(modstate=ModerationState.APPROVED, date__gt=datetime.now() - timedelta(days=7)).exclude(postedto__has_keys=allprovidernames).order_by('date'))
         if not len(articles):
             return