FIX: Don't override timezone on every visit of profile preferences

The timezone should only be initialized when there is no timezone stored in the DB yet. This also fixes an error that happened in dev mode whenever you switch between user preference tabs.

app/assets/javascripts/discourse/routes/preferences-profile.js.es6

@@ -3,8 +3,10 @@ import RestrictedUserRoute from "discourse/routes/restricted-user";
 export default RestrictedUserRoute.extend({
   showFooter: true,
   setupController(controller, model) {
-    model.user_option.timezone =
-      model.user_option.timezone || moment.tz.guess();
+    if (!model.user_option.timezone) {
+      Ember.set(model, "user_option.timezone", moment.tz.guess());
+    }
+
     controller.set("model", model);
   }
 });