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)

app/javascript/mastodon/features/direct_timeline/index.js

@@ -19,7 +19,6 @@ class DirectTimeline extends React.PureComponent {
 
   static propTypes = {
     dispatch: PropTypes.func.isRequired,
-    shouldUpdateScroll: PropTypes.func,
     columnId: PropTypes.string,
     intl: PropTypes.object.isRequired,
     hasUnread: PropTypes.bool,
@@ -71,7 +70,7 @@ class DirectTimeline extends React.PureComponent {
   }
 
   render () {
-    const { intl, hasUnread, columnId, multiColumn, shouldUpdateScroll } = this.props;
+    const { intl, hasUnread, columnId, multiColumn } = this.props;
     const pinned = !!columnId;
 
     return (
@@ -93,7 +92,6 @@ class DirectTimeline extends React.PureComponent {
           timelineId='direct'
           onLoadMore={this.handleLoadMore}
           emptyMessage={<FormattedMessage id='empty_column.direct' defaultMessage="You don't have any direct messages yet. When you send or receive one, it will show up here." />}
-          shouldUpdateScroll={shouldUpdateScroll}
         />
       </Column>
     );