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/components/status.js

@@ -90,6 +90,10 @@ export default class Status extends ImmutablePureComponent {
   }
 
   componentWillUnmount () {
+    if (this.props.intersectionObserverWrapper) {
+      this.props.intersectionObserverWrapper.unobserve(this.props.id, this.node);
+    }
+
     this.componentMounted = false;
   }