Add confirmation modal for pending users auto-approval

What does this MR do?

This is the final implementation MR for #293018 (closed). All relevant details can be found in the issue description.

Screenshots

	Modal

Testing scenarios

The testing scenarios are too many to be described here, and can be found in the specs (after visiting the URL in the link, wait a bit until the page scrolls to the specs -- seems there's a bug that slows this down)

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because _____.
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team