Provide a css rule for informational notifications

aka Notification::info()

public/css/icinga/main-content.less

@@ -41,6 +41,10 @@ img.icon {
   background-position: 1em center;
 }
 
+#notifications > li.info {
+  background-color: @colorFormNotificationInfo;
+}
+
 #notifications > li.warning {
   background-color: @colorWarningHandled;
 }