fix login page override css

just centers form, rather than allowing it to wrap weirdly

IPython/frontend/html/notebook/static/auth/css/override.css

@@ -3,7 +3,6 @@ This is only required when different pages style the same element differently. T
 a hack to deal with our current css styles and no new styling should be added in this file.*/
 
 #ipython-main-app {
-    height: 100px;
-    width: 350px;
     margin: 50px auto;
+    text-align: center;
 }