Prevent active admin from clobbering all jQuery UI dialog close buttons

Active Admin seems to now include some default styling for jQuery UI dialogs, which is mostly fine. But one of those changes hides the close button of the dialog, on every jQuery UI dialog, even when you want one.

This patch simply gives Active Admin created modal dialogs an `.active_admin_dialog` class, and namespaces the close button hiding CSS to just those dialogs.

Now apps that use their own jQuery UI dialogs can have have a close button on them.