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/direct_timeline/components/conversations_list.js

@@ -14,7 +14,6 @@ export default class ConversationsList extends ImmutablePureComponent {
     hasMore: PropTypes.bool,
     isLoading: PropTypes.bool,
     onLoadMore: PropTypes.func,
-    shouldUpdateScroll: PropTypes.func,
   };
 
   getCurrentIndex = id => this.props.conversations.findIndex(x => x.get('id') === id)