M.util.show_confirm_dialog should use new-style dialogues

This got missed in MDL-68409 and is another one that we can easily convert.

We should also look at replacing the YUI confirm dialogue (and others) but those are somewhat harder so probably belong in a separate issue.

Note: Ideally we should entirely deprecate this feature, but it is intrinsically linked to the confirm_action feature, which uses onclick handlers and we need to think about how weupdate this properly.