588575: Add missing cancel button on identity screens

What does this MR do and why?

This MR adds a "Cancel" button to the New Identity and Edit Identity forms within the Admin Area.

Currently, when an administrator navigates to Admin > Users > [User] > Identities > New/Edit, the form only provides a "Save" action. This deviates from GitLab's UI standards, where secondary "Cancel" actions are consistently provided to allow users to exit a form without submitting data.

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Related to #588575