Eager load event target authors whenever possible

What does this MR do?

This ensures that the "author" association of an event's "target" association is eager loaded whenever the "target" association defines an "author" association. This in turn solves the N+1 query problem we first tried to solve in !15788 (merged) but caused problems when displaying milestones as those don't define an "author" association.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• 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?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/41618

cc @stanhu @fjsanpedro