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/media_modal.js

@@ -20,8 +20,6 @@ const messages = defineMessages({
   next: { id: 'lightbox.next', defaultMessage: 'Next' },
 });
 
-export const previewState = 'previewMediaModal';
-
 export default @injectIntl
 class MediaModal extends ImmutablePureComponent {
 
@@ -37,10 +35,6 @@ class MediaModal extends ImmutablePureComponent {
     volume: PropTypes.number,
   };
 
-  static contextTypes = {
-    router: PropTypes.object,
-  };
-
   state = {
     index: null,
     navigationHidden: false,
@@ -98,16 +92,6 @@ class MediaModal extends ImmutablePureComponent {
   componentDidMount () {
     window.addEventListener('keydown', this.handleKeyDown, false);
 
-    if (this.context.router) {
-      const history = this.context.router.history;
-
-      history.push(history.location.pathname, previewState);
-
-      this.unlistenHistory = history.listen(() => {
-        this.props.onClose();
-      });
-    }
-
     this._sendBackgroundColor();
   }
 
@@ -131,14 +115,6 @@ class MediaModal extends ImmutablePureComponent {
   componentWillUnmount () {
     window.removeEventListener('keydown', this.handleKeyDown);
 
-    if (this.context.router) {
-      this.unlistenHistory();
-
-      if (this.context.router.history.location.state === previewState) {
-        this.context.router.history.goBack();
-      }
-    }
-
     this.props.onChangeBackgroundColor(null);
   }