Use custom modal instead of browser confirm

What does this MR do and why?

This overwrites Rails.confirm from @rails/ujs in order to show a modal instead of a Browser window.confirm.

Screenshots or screen recordings

Before	After
data-confirm-before	data-confirm-after

How to set up and validate locally

1. Enable the bootstrap_confirmation_modals feature flag.
2. Unprotect a protected branch.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.