Resolve "Add functionality to change what email address online actions commit using"

What does this MR do?

Follow-up to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21213/diffs#note_99488777 that fixes a small issue with the list of displayed commit emails, bearing https://gitlab.com/gitlab-org/gitlab-ce/issues/51235 in mind.

We should only allow verified emails that are not the primary address to be explicitly selected.

This shows additional commits because !21213 (closed) isn't merged yet. In practice, only 43a9b530 is unreviewed.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #23986 (closed)