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/containers/scroll_container.js

@@ -0,0 +1,18 @@
+import { ScrollContainer as OriginalScrollContainer } from 'react-router-scroll-4';
+
+// ScrollContainer is used to automatically scroll to the top when pushing a
+// new history state and remembering the scroll position when going back.
+// There are a few things we need to do differently, though.
+const defaultShouldUpdateScroll = (prevRouterProps, { location }) => {
+  // If the change is caused by opening a modal, do not scroll to top
+  return !(location.state?.mastodonModalKey && location.state?.mastodonModalKey !== prevRouterProps?.location?.state?.mastodonModalKey);
+};
+
+export default
+class ScrollContainer extends OriginalScrollContainer {
+
+  static defaultProps = {
+    shouldUpdateScroll: defaultShouldUpdateScroll,
+  };
+
+}