feat(analytics): AB testing

User model update for AB tests, and the first AB test to start Sept 12

closes #7984

test/helpers/mongo.js

@@ -1,5 +1,6 @@
 import mongoose from 'mongoose';
 import { TAVERN_ID } from '../../website/server/models/group';
+import { get } from 'lodash';
 
 // Useful for checking things that have been deleted,
 // but you no longer have access to,
@@ -18,6 +19,20 @@ export async function checkExistence (collectionName, id) {
   });
 }
 
+// Obtain a property from the database. Useful if the property is private
+// and thus unavailable to the client
+export async function getProperty (collectionName, id, path) {
+  return new Promise((resolve, reject) => {
+    let collection = mongoose.connection.db.collection(collectionName);
+
+    collection.find({_id: id}, {[path]: 1}).limit(1).toArray((findError, docs) => {
+      if (findError) return reject(findError);
+
+      resolve(get(docs[0], path));
+    });
+  });
+}
+
 // Specifically helpful for the GET /groups tests,
 // resets the db to an empty state and creates a tavern document
 export async function resetHabiticaDB () {