Add Fork/Cancel confirmation to "Replace"/"Delete" buttons

What does this MR do?

• Add Fork/Cancel confirmation to "Replace"/"Delete" buttons (see blob:show, blame:show)

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Previously when needing to fork, clicking on "Replace"/"Delete" would fork immediately. This is inconsistent with what we decided to move to with the "Edit" button that has a verification Fork/Cancel button combo , https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10233

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #30637 (closed)