Add a check for incorrect Resource URL

While rewriting the sysadmin documentation, I considered how to convey the trick most experienced Civi folk know: "If you can't see arrow icons in your Civi menus, your Resource URL is messed up."

From a usability perspective, I realized this made more sense as a system check than a troubleshooting step. I wrote a patch to check that the Resource URL is configured correctly.  In theory, with the `[civicrm.root]` token this should never get triggered, but I somehow suspect it will.