Update cms.modal.js

The CMS modal was not displaying correctly in Firefox or Chrome. $(window).height() posed a problem when the height of the content in the CMS modal was greater than the screen height. Replacing $(window).height() with screen.height fixed the problem. This problem could also present itself when the content width is greater than screen width - thus $(window).width() was changed to screen.width.
Merge pull request #2599 from gentleShark/develop

Update cms.modal.js