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: gitlab-ee!14416 (merged)
Does this MR meet the acceptance criteria?
- 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