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/ui/components/audio_modal.js

@@ -4,7 +4,6 @@ import PropTypes from 'prop-types';
 import Audio from 'mastodon/features/audio';
 import { connect } from 'react-redux';
 import ImmutablePureComponent from 'react-immutable-pure-component';
-import { previewState } from './video_modal';
 import Footer from 'mastodon/features/picture_in_picture/components/footer';
 
 const mapStateToProps = (state, { statusId }) => ({
@@ -25,32 +24,6 @@ class AudioModal extends ImmutablePureComponent {
     onChangeBackgroundColor: PropTypes.func.isRequired,
   };
 
-  static contextTypes = {
-    router: PropTypes.object,
-  };
-
-  componentDidMount () {
-    if (this.context.router) {
-      const history = this.context.router.history;
-
-      history.push(history.location.pathname, previewState);
-
-      this.unlistenHistory = history.listen(() => {
-        this.props.onClose();
-      });
-    }
-  }
-
-  componentWillUnmount () {
-    if (this.context.router) {
-      this.unlistenHistory();
-
-      if (this.context.router.history.location.state === previewState) {
-        this.context.router.history.goBack();
-      }
-    }
-  }
-
   render () {
     const { media, accountStaticAvatar, statusId, onClose } = this.props;
     const options = this.props.options || {};