[1.x] [extensibility] refactor(core, flags): improve & use extensibil…

…ity of `CommentPost` & `Post` (#4047)

* refactor(core): improve extensibility of `CommentPost`

* refactor(core): rename method to more appropriate name

* refactor(core): further improve extensibility of `CommentPost`

* refactor(core): improve extensibility of `Post`

* refactor(flags): use new extensibility for flagged posts

js/src/forum/addFlagsToPosts.js

@@ -75,23 +75,25 @@ export default function () {
     return items;
   };
 
-  extend(Post.prototype, 'content', function (vdom) {
+  extend(Post.prototype, 'viewItems', function (items) {
     const post = this.attrs.post;
     const flags = post.flags();
 
     if (!flags.length) return;
 
     if (post.isHidden()) this.revealContent = true;
 
-    vdom.unshift(
+    items.add(
+      'flagged',
       <div className="Post-flagged">
         <div className="Post-flagged-flags">
           {flags.map((flag) => (
             <div className="Post-flagged-flag">{this.flagReason(flag)}</div>
           ))}
         </div>
         <div className="Post-flagged-actions">{this.flagActionItems().toArray()}</div>
-      </div>
+      </div>,
+      110
     );
   });