Fix import no longer mapping users as admin

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25346

• Users were no longer mapped in notes, due to some author_id overriding.
• When the importer is not an admin, we'll still map users anyway (for anything that wasn't a note). I guess we shouldn't be doing that.
• We'll first find users by their username, but we should be giving priority to the email, as it's a more accurate match. (dev to gitlab.com problem).

• Changelog entry added
• 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?