DEV: prevents post.siteSettings computed property to be overridden (#7941)

This happens when loading a post from a json object and is a behavior which will be impossble in future Ember updates.

app/assets/javascripts/discourse/models/post.js.es6

@@ -11,12 +11,18 @@ import { userPath } from "discourse/lib/url";
 import Composer from "discourse/models/composer";
 
 const Post = RestModel.extend({
-  @computed()
-  siteSettings() {
   // TODO: Remove this once one instantiate all `Discourse.Post` models via the store.
+  siteSettings: Ember.computed({
+    get() {
       return Discourse.SiteSettings;
     },
 
+    // prevents model created from json to overridde this property
+    set() {
+      return Discourse.SiteSettings;
+    }
+  }),
+
   @computed("url")
   shareUrl(url) {
     const user = Discourse.User.current();