Clean up, don't use mixin

PhpStorm/WebStorm doesn't like the mixin syntax, and it's clearer to just use Object.assign.

js/lib/models/Forum.js

@@ -1,9 +1,7 @@
 import Model from 'flarum/Model';
 import mixin from 'flarum/utils/mixin';
 
-export default class Forum extends mixin(Model, {
-  canStartDiscussion: Model.attribute('canStartDiscussion')
-}) {
+export default class Forum extends Model {
   apiEndpoint() {
     return '/forum';
   }