Close stale branches

The following branches are very stale and I believe should be deleted:

- [ ] binh_sidebar
- [ ] importer-cnx-ver1
- [ ] ReactSidebar

The following branches are somewhat stale but don't seem to have any new content:
- [ ] a11y
- [ ] 210-update-sidebar-links

I'm not sure about the status of this branch:
- ReferenceManager