Backport of #21230
The problem was that many PR review components loaded by `Show more`
received the same ID as previous batches, which confuses browsers (when
clicked). All such occurrences should now be fixed.
Additionally improved the background of the `viewed` checkbox.
Fixes#21228.
Fixes#20681.
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>