Mixed type field for A/B testing (#8302)

* feat(AB-testing): mixed type field

* fix(AB-testing): lint errors

* fix(AB-testing): allow client access to _ABtests

* Revert "fix(AB-testing): allow client access to _ABtests"

This reverts commit 25832365ba.

* fix(AB-testing): preview check on server

* refactor(AB-testing): add comments

website/server/libs/analyticsService.js

@@ -7,6 +7,7 @@ import useragent from 'useragent';
 import {
   each,
   omit,
+  toArray,
 } from 'lodash';
 import { content as Content } from '../../common';
 
@@ -90,6 +91,9 @@ let _formatUserData = (user) => {
   if (user._ABtest) {
     properties.ABtest = user._ABtest;
   }
+  if (user._ABtests) {
+    properties.ABtests = toArray(user._ABtests);
+  }
 
   if (user.registeredThrough) {
     properties.registeredPlatform = user.registeredThrough;