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/favourited_statuses/index.js

@@ -27,7 +27,6 @@ class Favourites extends ImmutablePureComponent {
 
   static propTypes = {
     dispatch: PropTypes.func.isRequired,
-    shouldUpdateScroll: PropTypes.func,
     statusIds: ImmutablePropTypes.list.isRequired,
     intl: PropTypes.object.isRequired,
     columnId: PropTypes.string,
@@ -68,7 +67,7 @@ class Favourites extends ImmutablePureComponent {
   }, 300, { leading: true })
 
   render () {
-    const { intl, shouldUpdateScroll, statusIds, columnId, multiColumn, hasMore, isLoading } = this.props;
+    const { intl, statusIds, columnId, multiColumn, hasMore, isLoading } = this.props;
     const pinned = !!columnId;
 
     const emptyMessage = <FormattedMessage id='empty_column.favourited_statuses' defaultMessage="You don't have any favourite toots yet. When you favourite one, it will show up here." />;
@@ -93,7 +92,6 @@ class Favourites extends ImmutablePureComponent {
           hasMore={hasMore}
           isLoading={isLoading}
           onLoadMore={this.handleLoadMore}
-          shouldUpdateScroll={shouldUpdateScroll}
           emptyMessage={emptyMessage}
           bindToDocument={!multiColumn}
         />