Refactor gdAlert

Hello,

Just a general update to the gdAlert function.

* Move CSS out to res.css (cleanup styles, seems styles and markup may have got out of sync at some point)
* Use jQuery to calculate page centre (don't need to worry about scroll anymore as position:fixed is in use)
* Fix populateContainer being called twice
* Fix multiple copies of alert_message and alert_message_background being added to the DOM
* Clear out some comments

Thanks,
Carl