Fixed issue boards scrolling with a lot of lists & issues

## What does this MR do?

When an issue board has a lot of lists & each list has a lot of issues scrolling can be tricky. This changes that by making the element that scrolls the `board-list` div. This is ok because sorting inside each list isn't possible, it will need re-thinking when sorting is possible but this fixes the bug for now.

See merge request !7748