Require all reviews to be approved

This way we ensure one set of reviews doesn't override a different set
of reviews by someone else. All reviewers should have their concerns
addressed before merging.

.mergify.yml

@@ -3,6 +3,9 @@ pull_request_rules:
     conditions:
       - base=master
       - "#approved-reviews-by>=1"
+      - "#review-requested=0"
+      - "#changes-requested-reviews-by=0"
+      - "#commented-reviews-by=0"
       - label=ready-to-merge
       - label!=hold-off-merging
     actions: