Add authenticated photo uploads and GitHub Actions publishing

scripts/photo-manifest.py

@@ -0,0 +1,10 @@
+"""Receipt of the photos actually rendered by this build."""
+import json
+import os
+from pathlib import Path
+import re
+
+photos = sorted(set(re.findall(r'id="photo-([a-f0-9]{64})"', Path('_site/art/index.html').read_text())))
+Path('_site/photo-publication.json').write_text(json.dumps({
+    'commit': os.environ['GITHUB_SHA'], 'photos': photos,
+}) + '\n')