migrate-group: Flag un-migrated objects for manual review

I'd been doing this manually while reviewing the previous sync logs, as
before the layout change the sync just migrated everything even though
some objects would be no longer accessible.  Nicer to have it built-in
as part of the rest of the manual steps checklist.

scripts/migrate-group

@@ -127,7 +127,17 @@ async function syncData({dryRun = true, group}) {
     ]
   });
 
+  // Discover files to consider for manual review
+  const unsynced = (await s3ListObjects({group})).filter(
+    key => !key.endsWith(".json")
+        && !key.endsWith(".md")
+        && key !== "group-overview.md"
+        && key !== "group-logo.png"
+  );
+
   console.groupEnd();
+
+  return unsynced.map(key => `Investigate unsynced object s3://${group.bucket}/${key}`);
 }
 
 
@@ -148,6 +158,16 @@ async function s3Sync({dryRun = true, group, prefix = "", filters = []}) {
 }
 
 
+async function s3ListObjects({group}) {
+  const client = new S3.S3Client();
+
+  return await collate(
+    S3.paginateListObjectsV2({client}, {Bucket: group.bucket}),
+    page => page.Contents.map(object => object.Key),
+  );
+}
+
+
 async function updateServerPolicies({dryRun = true, oldBucket}) {
   const policyFiles = [
     "aws/iam/policy/NextstrainDotOrgServerInstance.json",