Change uploader to actor

Class User doesn't have field `uploader` any more, instead there
`actor`. Fix several uses of `uploader` to `actor`.

mediagoblin/user_pages/views.py

@@ -656,7 +656,7 @@ def processing_panel(request, page, url_user):
             request, 'mediagoblin.user_pages.user_home',
             user=user.username)
     # Get media entries which are in-processing
-    entries = (MediaEntry.query.filter_by(uploader=user.id)
+    entries = (MediaEntry.query.filter_by(actor=user.id)
             .order_by(MediaEntry.created.desc()))
 
     try: