Fix hide and show media button on admin page (#6347)

app/javascript/packs/admin.js

@@ -28,13 +28,13 @@ delegate(document, batchCheckboxClassName, 'change', () => {
 });
 
 delegate(document, '.media-spoiler-show-button', 'click', () => {
-  [].forEach.call(document.querySelectorAll('.activity-stream .media-spoiler-wrapper'), (content) => {
+  [].forEach.call(document.querySelectorAll('button.media-spoiler'), (element) => {
-    content.classList.add('media-spoiler-wrapper__visible');
+    element.click();
   });
 });
 
 delegate(document, '.media-spoiler-hide-button', 'click', () => {
-  [].forEach.call(document.querySelectorAll('.activity-stream .media-spoiler-wrapper'), (content) => {
+  [].forEach.call(document.querySelectorAll('.spoiler-button.spoiler-button--visible button'), (element) => {
-    content.classList.remove('media-spoiler-wrapper__visible');
+    element.click();
   });
 });