Show confirmation modal before deleting account

What does this MR do?

Add a new modal to confirm account deletion.

Screenshots

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Squashed related commits together

What are the relevant issue numbers?

closes https://gitlab.com/gitlab-org/gitlab-ce/issues/13482

fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/38754