Fix Back button going outside the app (#24835)

app/javascript/mastodon/components/column_back_button.jsx

@@ -21,7 +21,10 @@ export default class ColumnBackButton extends React.PureComponent {
 
     if (onClick) {
       onClick();
-    } else if (window.history && window.history.state) {
+    } 
+    // Check if there is a previous page in the app to go back to per https://stackoverflow.com/a/70532858/9703201
+    // When upgrading to V6, check `location.key !== 'default'` instead per https://github.com/remix-run/history/blob/main/docs/api-reference.md#location
+    else if (router.location.key) {
       router.history.goBack();
     } else {
       router.history.push('/');