Reuse authors when rendering event Atom feeds

What does this MR do?

This changes the code used for rendering events atom feeds so we don't execute N+1 queries just to get author Emails. See the commit message for more details.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/36878