Centre the sponsor line in the footer, cache-bust the stylesheet

css/site.css

@@ -890,7 +890,9 @@ html.js [data-rv].rv-in { opacity: 1; transform: none; }
 }
 .sponsor a:hover { color: var(--navy); }
 .sponsor-foot {
-  margin: 10px 0 0;
+  max-width: var(--max);
+  margin: 22px auto 0;
+  text-align: center;
   font-size: var(--step-sm);
   color: rgba(246, 236, 218, 0.72);
 }