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/favourites/index.js

@@ -27,7 +27,6 @@ class Favourites extends ImmutablePureComponent {
   static propTypes = {
     params: PropTypes.object.isRequired,
     dispatch: PropTypes.func.isRequired,
-    shouldUpdateScroll: PropTypes.func,
     accountIds: ImmutablePropTypes.list,
     multiColumn: PropTypes.bool,
     intl: PropTypes.object.isRequired,
@@ -50,7 +49,7 @@ class Favourites extends ImmutablePureComponent {
   }
 
   render () {
-    const { intl, shouldUpdateScroll, accountIds, multiColumn } = this.props;
+    const { intl, accountIds, multiColumn } = this.props;
 
     if (!accountIds) {
       return (
@@ -74,7 +73,6 @@ class Favourites extends ImmutablePureComponent {
 
         <ScrollableList
           scrollKey='favourites'
-          shouldUpdateScroll={shouldUpdateScroll}
           emptyMessage={emptyMessage}
           bindToDocument={!multiColumn}
         >