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/community_timeline/index.js

@@ -41,7 +41,6 @@ class CommunityTimeline extends React.PureComponent {
 
   static propTypes = {
     dispatch: PropTypes.func.isRequired,
-    shouldUpdateScroll: PropTypes.func,
     columnId: PropTypes.string,
     intl: PropTypes.object.isRequired,
     hasUnread: PropTypes.bool,
@@ -103,7 +102,7 @@ class CommunityTimeline extends React.PureComponent {
   }
 
   render () {
-    const { intl, shouldUpdateScroll, hasUnread, columnId, multiColumn, onlyMedia } = this.props;
+    const { intl, hasUnread, columnId, multiColumn, onlyMedia } = this.props;
     const pinned = !!columnId;
 
     return (
@@ -127,7 +126,6 @@ class CommunityTimeline extends React.PureComponent {
           timelineId={`community${onlyMedia ? ':media' : ''}`}
           onLoadMore={this.handleLoadMore}
           emptyMessage={<FormattedMessage id='empty_column.community' defaultMessage='The local timeline is empty. Write something publicly to get the ball rolling!' />}
-          shouldUpdateScroll={shouldUpdateScroll}
           bindToDocument={!multiColumn}
         />
       </Column>