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/column_collapsable.js

@@ -36,7 +36,7 @@ class ColumnCollapsable extends React.PureComponent {
         </div>
 
         <div className='column-collapsable__content' style={{ height: `${fullHeight}px` }}>
-          {children}
+          {!collapsed && children}
         </div>
       </div>
     );