minor notebook logging changes

log refer only on 4XX, and never log user agent

This function was ported from nbviewer, and some of the choices were not quite right for the notebook server.

IPython/html/log.py

@@ -37,14 +37,10 @@ def log_request(handler):
         request_time=request_time,
     )
     msg = "{status} {method} {uri} ({ip}) {request_time:.2f}ms"
-    if status >= 300:
-        # log referers on redirects
+    if status >= 400:
+        # log bad referers
         ns['referer'] = request.headers.get('Referer', 'None')
         msg = msg + ' referer={referer}'
-    if status >= 400:
-        # log user agent for failed requests
-        ns['agent'] = request.headers.get('User-Agent', 'Unknown')
-        msg = msg + ' user-agent={agent}'
     if status >= 500 and status != 502:
         # log all headers if it caused an error
         log_method(json.dumps(request.headers, indent=2))