Our API currently [offers the ability to unapprove a MR](https://docs.gitlab.com/ee/api/merge_request_approvals.html#unapprove-merge-request) but it's limited to removing _only_ the approval (if it exists) for the user associated to the API call. We should create a new API endpoint, perhaps `/projects/:id/merge_requests/:merge_request_iid/unapprove_all` to offer to ability to remove all approvals, ensuring it's only available to higher privileged API tokens.