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/features/ui/components/modal_root.js

@@ -5,7 +5,8 @@ import OnboardingModal from './onboarding_modal';
 import VideoModal from './video_modal';
 import BoostModal from './boost_modal';
 import ConfirmationModal from './confirmation_modal';
-import { TransitionMotion, spring } from 'react-motion';
+import TransitionMotion from 'react-motion/lib/TransitionMotion';
+import spring from 'react-motion/lib/spring';
 
 const MODAL_COMPONENTS = {
   'MEDIA': MediaModal,