Support for multiple required approvers in batch changes

Created by: eseliger

Currently, we derive the approval state of a changeset from the events solely: if the last event was approved and no one requested changes, we treat it as approved. Instead, for GitHub and GitLab we would need to take the repo settings into account as well: If there is a "multiple approvers required" rule in effect, we'd need to take that into consideration as well. As the current algorithm purely works based on events, that might be hard to achieve though. Also, when the repo settings change, would that invalidate all approved states from the past? Ideally not.

Impacted customers

See: https://sourcegraph2020.lightning.force.com/lightning/r/Product_Gap__c/a1B5b00000LAJEPEA5/view