Allow collapsing all issue boards

What does this MR do?

Fixes #40630 (closed)

Allows collapsing all issue boards via a caret icon, and dragging any collapsed or expanded issue board by its header.

Related EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/14416

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

app/assets/javascripts/boards/components/board.js

@@ -84,7 +84,7 @@ export default Vue.extend({
       group: 'boards',
       draggable: '.is-draggable',
       handle: '.js-board-handle',
-      onEnd: function onEnd(e) {
+      onEnd(e) {
         sortableEnd();
 
         const sortable = this;