Homepage (TDD slice 7): fix the /admin/ path + status.json perms

Two real bugs found on the first production deploy:
- /admin/ 404'd: file_server looked for /srv/homepage/admin/ (a dir), but the
  page is admin.html. Add '@admin rewrite * /admin.html' so the /admin/ path
  serves admin.html.
- The pages used relative refs (style.css, app.js, services.json, status.json),
  which from /admin/ resolved to /admin/... (404). Add <base href="/"> so all
  refs resolve to the site root on both pages.
- status.json was written 0600 (root); the checker now chmods it 0644 so Caddy
  can serve it regardless of its user.

scripts/homepage-status.py

@@ -71,6 +71,7 @@ def main(argv):
         with os.fdopen(fd, "w") as f:
             json.dump(doc, f)
         os.replace(tmp, out_path)
+        os.chmod(out_path, 0o644)  # world-readable so Caddy can serve it
     except Exception:
         if os.path.exists(tmp):
             os.unlink(tmp)