Add MDNlinks to Challenge scheme so they can display on bonfire views.

models/Challenge.js

@@ -16,7 +16,8 @@ var challengeSchema = new mongoose.Schema({
   tests: Array,
   challengeSeed: Array,
   completionMessage: String, // Congratulations! You've finished our HTML and CSS track!
-  challengeType: Number // 0 = html, 1 = javascript only, 2 = video, 3 = zipline, 4 = basejump
+  challengeType: Number, // 0 = html, 1 = javascript only, 2 = video, 3 = zipline, 4 = basejump
+  MDNlinks: Array
 });
 
 module.exports = mongoose.model('Challenge', challengeSchema);