define a header that enables 503 pages to disengage applySafeRedirector.

war/src/main/webapp/scripts/hudson-behavior.js

@@ -2132,7 +2132,7 @@ function applySafeRedirector(url) {
         new Ajax.Request(url, {
             method: "get",
             onFailure: function(rsp) {
-                if(rsp.status==503) {
+                if(rsp.status==503 && rsp.getHeader("X-Jenkins-Interactive")==null) {
                   // redirect as long as we are still loading
                   window.setTimeout(statusChecker,5000);
                 } else {