Add registry delete confirmation

Imported from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18568

What does this MR do?

• Adds JavaScript confirmation dialogs for the "delete repository" and "delete tag" actions in the container registry.
• Also adds a warning that clicking "delete tag" might delete other tags as well (#21405 (closed))

Why was this MR needed?

• This is a destructive operation
• Similar dialogs exist for deleting e.g. comments
• Issue made more serious because of #21405 (closed)

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

#21405 (closed)