Replace haml confirm_modal with GlModal

Background

As part of our effort to replace bootstrap modals, we should replace the existing haml confirm_modal with a vue component, utilising GlModal

Implementation

• Mark the existing js-confirm-modal* as js-legacy-confirm-modal*
• Add a vue component implementing the confirm_modal
• Add an init script to mount the modal app
• Add tests for the new component

Related to #254274 (closed).