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/list_timeline/index.js

@@ -41,7 +41,6 @@ class ListTimeline extends React.PureComponent {
   static propTypes = {
     params: PropTypes.object.isRequired,
     dispatch: PropTypes.func.isRequired,
-    shouldUpdateScroll: PropTypes.func,
     columnId: PropTypes.string,
     hasUnread: PropTypes.bool,
     multiColumn: PropTypes.bool,
@@ -142,7 +141,7 @@ class ListTimeline extends React.PureComponent {
   }
 
   render () {
-    const { shouldUpdateScroll, hasUnread, columnId, multiColumn, list, intl } = this.props;
+    const { hasUnread, columnId, multiColumn, list, intl } = this.props;
     const { id } = this.props.params;
     const pinned = !!columnId;
     const title  = list ? list.get('title') : id;
@@ -207,7 +206,6 @@ class ListTimeline extends React.PureComponent {
           timelineId={`list:${id}`}
           onLoadMore={this.handleLoadMore}
           emptyMessage={<FormattedMessage id='empty_column.list' defaultMessage='There is nothing in this list yet. When members of this list post new statuses, they will appear here.' />}
-          shouldUpdateScroll={shouldUpdateScroll}
           bindToDocument={!multiColumn}
         />
       </Column>