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/lists/index.js

@@ -48,7 +48,7 @@ class Lists extends ImmutablePureComponent {
   }
 
   render () {
-    const { intl, shouldUpdateScroll, lists, multiColumn } = this.props;
+    const { intl, lists, multiColumn } = this.props;
 
     if (!lists) {
       return (
@@ -68,7 +68,6 @@ class Lists extends ImmutablePureComponent {
 
         <ScrollableList
           scrollKey='lists'
-          shouldUpdateScroll={shouldUpdateScroll}
           emptyMessage={emptyMessage}
           prepend={<ColumnSubheading text={intl.formatMessage(messages.subheading)} />}
           bindToDocument={!multiColumn}