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/notifications/index.js

@@ -74,7 +74,6 @@ class Notifications extends React.PureComponent {
     notifications: ImmutablePropTypes.list.isRequired,
     showFilterBar: PropTypes.bool.isRequired,
     dispatch: PropTypes.func.isRequired,
-    shouldUpdateScroll: PropTypes.func,
     intl: PropTypes.object.isRequired,
     isLoading: PropTypes.bool,
     isUnread: PropTypes.bool,
@@ -176,7 +175,7 @@ class Notifications extends React.PureComponent {
   };
 
   render () {
-    const { intl, notifications, shouldUpdateScroll, isLoading, isUnread, columnId, multiColumn, hasMore, numPending, showFilterBar, lastReadId, canMarkAsRead, needsNotificationPermission } = this.props;
+    const { intl, notifications, isLoading, isUnread, columnId, multiColumn, hasMore, numPending, showFilterBar, lastReadId, canMarkAsRead, needsNotificationPermission } = this.props;
     const pinned = !!columnId;
     const emptyMessage = <FormattedMessage id='empty_column.notifications' defaultMessage="You don't have any notifications yet. When other people interact with you, you will see it here." />;
 
@@ -227,7 +226,6 @@ class Notifications extends React.PureComponent {
         onLoadPending={this.handleLoadPending}
         onScrollToTop={this.handleScrollToTop}
         onScroll={this.handleScroll}
-        shouldUpdateScroll={shouldUpdateScroll}
         bindToDocument={!multiColumn}
       >
         {scrollableContent}