[#1672] assign tasks.id = tasks._id for challenges, fixes html ids & for lables

src/models/challenge.js

@@ -39,6 +39,11 @@ ChallengeSchema.methods.toJSON = function(){
   var doc = this.toObject();
   doc.memberCount = doc.members ? _.size(doc.members) : doc.memberCount; // @see pre('save') comment above
   doc._isMember = this._isMember;
+  _.each(['habits','dailys','todos','rewards'], function(type){
+    _.each(doc[type],function(task){
+      task.id = task._id;
+    })
+  })
   return doc;
 }