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/models/user/schema.js

@@ -527,7 +527,10 @@ let schema = new Schema({
     return {};
   }},
   pushDevices: [PushDeviceSchema],
-  _ABtest: {type: String},
+  _ABtest: {type: String}, // deprecated. Superseded by _ABtests
+  _ABtests: {type: Schema.Types.Mixed, default: () => {
+    return {};
+  }},
   webhooks: [WebhookSchema],
   loginIncentives: {type: Number, default: 0},
 }, {