Unobserve status on unmount (#4013)

This fixes a warning on status unmounting (e.g. deletion).

This also resets IntersectionObserverWrapper on disconnect to avoid `unobserve()` calls
which has bug in Edge.

app/javascript/mastodon/features/ui/util/intersection_observer_wrapper.js

@@ -37,9 +37,18 @@ class IntersectionObserverWrapper {
     }
   }
 
+  unobserve (id, node) {
+    if (this.observer) {
+      delete this.callbacks[id];
+      this.observer.unobserve(node);
+    }
+  }
+
   disconnect () {
     if (this.observer) {
+      this.callbacks = {};
       this.observer.disconnect();
+      this.observer = null;
     }
   }