Resolves umputun#559

Adds .comment_highlighting class after scrolling to comment.

frontend/app/components/comment/_theme/_dark/comment_theme_dark.scss

@@ -95,4 +95,8 @@
       }
     }
   }
+
+  &.comment_highlighting {
+    background: var(--color19);
+  }
 }

frontend/app/components/comment/_theme/_light/comment_theme_light.scss

@@ -95,4 +95,8 @@
       }
     }
   }
+
+  &.comment_highlighting {
+    background: var(--color4);
+  }
 }

frontend/app/components/root/root.tsx

@@ -136,6 +136,10 @@ export class Root extends Component<Props, State> {
       if (comment) {
         setTimeout(() => {
           postMessage({ scrollTo: comment.getBoundingClientRect().top });
+          comment.classList.add('comment_highlighting');
+          setTimeout(() => {
+            comment.classList.remove('comment_highlighting');
+          }, 5e3);
         }, 500);
       }
     }