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/pinned_statuses/index.js

@@ -24,7 +24,6 @@ class PinnedStatuses extends ImmutablePureComponent {
 
   static propTypes = {
     dispatch: PropTypes.func.isRequired,
-    shouldUpdateScroll: PropTypes.func,
     statusIds: ImmutablePropTypes.list.isRequired,
     intl: PropTypes.object.isRequired,
     hasMore: PropTypes.bool.isRequired,
@@ -44,7 +43,7 @@ class PinnedStatuses extends ImmutablePureComponent {
   }
 
   render () {
-    const { intl, shouldUpdateScroll, statusIds, hasMore, multiColumn } = this.props;
+    const { intl, statusIds, hasMore, multiColumn } = this.props;
 
     return (
       <Column bindToDocument={!multiColumn} icon='thumb-tack' heading={intl.formatMessage(messages.heading)} ref={this.setRef}>
@@ -53,7 +52,6 @@ class PinnedStatuses extends ImmutablePureComponent {
           statusIds={statusIds}
           scrollKey='pinned_statuses'
           hasMore={hasMore}
-          shouldUpdateScroll={shouldUpdateScroll}
           bindToDocument={!multiColumn}
         />
       </Column>