Allow all approvals for an MR to be removed via API

Our API currently offers the ability to unapprove a MR 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.