[#1729] move most chal-user syncing logic to their models rather than

controllers. don't sync task.value or task.history over to user on
challenge.join()! some work-arounds for the task._id/id mismatch (still
not perfect, need to work on some more)

src/models/task.js

@@ -38,4 +38,11 @@ var TaskSchema = new Schema({
   _id: false
 });
 
+/**
+ * Workaround for bug when _id & id were out of sync, we can remove this after challenges has been running for a while
+ */
+TaskSchema.post('init', function(doc){
+  if (!doc.id && doc._id) doc.id = doc._id;
+})
+
 module.exports.schema = TaskSchema;