[#1829] sort challenge-box by creation date

src/controllers/groups.js

@@ -33,7 +33,11 @@ var populateQuery = function(type, q){
   else
     q.populate(guildPopulate);
   q.populate('invites', nameFields);
-  q.populate('challenges', challengeFields);
+  q.populate({
+    path: 'challenges',
+    select: challengeFields,
+    options: {sort: [['timestamp',-1]]}
+  });
   return q;
 }