Add unread notification markers (#14818)

* Add unread notification markers

Fixes #14804

* Allow IntersectionObserverArticle's children to be updated

app/javascript/styles/mastodon/components.scss

@@ -7015,3 +7015,22 @@ noscript {
     }
   }
 }
+
+.notification,
+.status__wrapper {
+  position: relative;
+
+  &.unread {
+    &::before {
+      content: "";
+      position: absolute;
+      top: 0;
+      left: 0;
+      pointer-events: 0;
+      width: 100%;
+      height: 100%;
+      border-left: 2px solid $highlight-text-color;
+      pointer-events: none;
+    }
+  }
+}