Swap from using native confirm() to using an OOUI dialog when cancelling

Relies on:
* I292fb34d in OOjs UI to add the confirmation dialog
** I67329820 in MediaWiki core to use the messages added in OOjs UI
** I38f5bb63 in VisualEditor core to register the confirmation dialog

Bug: 50955
Change-Id: I98f9a03d780556b360b57c018c05a27cc1b3862e