Repo mirror: Add delete confirmation prompt
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### Problem to solve
Users can easily accidentally delete their repo mirror
### Intended users
All owners and maintainers.
### Further details
Prompted by customer report that they accidentally deleted their repo mirror: https://gitlab.zendesk.com/agent/tickets/124397
### Proposal
While we have a delete button:

we should add a prompt to have users confirm that they want to delete the repository mirror.
### What does success look like, and how can we measure that?
Users don't accidentally delete repo mirrors.
issue