Performance improvements (#3168)

* refactor(components/status_list): Avoid quering scrollTop if not necessary

* refactor(components/dropdown_menu): Do not render items if not expanded

* refactor: Cherry-pick react-motion imports

* refactor(compose/privacy_dropdown): Do not render options if not open

* refactor(components/column_collapsable): Do not render children if collapsed

app/javascript/mastodon/components/collapsable.js

@@ -1,5 +1,6 @@
 import React from 'react';
-import { Motion, spring } from 'react-motion';
+import Motion from 'react-motion/lib/Motion';
+import spring from 'react-motion/lib/spring';
 import PropTypes from 'prop-types';
 
 const Collapsable = ({ fullHeight, isVisible, children }) => (