Migrate IDE to use GlModal for discarding changes

What does this MR do and why?

Resolves #344173 (closed) (part of epic &6994 (closed))

Replace data-confirm modals in app/views/projects/_commit_button.html.haml with the implementation using GlModal.

Screenshots or screen recordings

before	after

Note: the changes are behind feature flag bootstrap_confirmation_modals

MR acceptance checklist

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