EE destroy repo mirrors

What does this MR do?

CE port: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27087

destroy repo mirrors when the delete button is clicked.

it is important to destroy instead of disable because it opaquely leaves sensitive data in the DB that could worsen a breach.

Use _destroy nested attribute instead of enabled for push mirrors.

Call remove_import_data after saving a project if its pull mirror is disabled.

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Because of remove_import_data if mirror? == false on save, we can no longer show users their public ssh key before they have saved the mirror.

We now use the same UX as push mirrors and provide the ssh public key through a copy to clipboard button in the mirrors table.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/54574

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer