Add a placeholder server/skin/custom.js to prevent one error in JS console

Add a placeholder server/skin/custom.js to prevent one error in JS console

custom.js enables you to override the settings in the stock constants.js
to skin the behaviour of your staff client at your installation, without
being subject to grief at upgrade time.

Not having a custom.js file at all was, however, causing errors to be generated
in the JS console as XULRunner tried hard to parse the 404 message as
JavaScript. I suppose one could have changed the 404 message to valid
JavaScript... naw, that would be evil.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@18068 dcc99617-32d9-48b4-a31d-7c20da2025e4