Add Dismiss stale pull request approvals when new commits are pushed equivalent option

[yuichielectric]

GitHub's PR approval is given for a specific commit. Therefore, it can happen that the same person 'request change' once for the same PR and then 'approve' it. Therefore, I think there should be an option to determine whether the latest commit has been approved or not. The logic right now is that if a reviewer have approved once in the past, they're considered to have approved.