Improve modal flow and back button handling (#16499)

* Refactor shouldUpdateScroll passing

So far, shouldUpdateScroll has been manually passed down from the very top of
the React component hierarchy even though it is a static function common to
all ScrollContainer instances, so replaced that with a custom class extending
ScrollContainer.

* Generalize “press back to close modal” to any modal and to public pages

* Fix boost confirmation modal closing media modal

app/javascript/mastodon/features/ui/containers/modal_container.js

@@ -3,8 +3,8 @@ import { closeModal } from '../../../actions/modal';
 import ModalRoot from '../components/modal_root';
 
 const mapStateToProps = state => ({
-  type: state.get('modal').modalType,
-  props: state.get('modal').modalProps,
+  type: state.getIn(['modal', 0, 'modalType'], null),
+  props: state.getIn(['modal', 0, 'modalProps'], {}),
 });
 
 const mapDispatchToProps = dispatch => ({