WIP: Resolve "Avatars make blame page slow"

What does this MR do?

Fetches avatar icons on the blame page with a single query rather than a query for each commit.

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

Why was this MR needed?

Improves blame page performance, esp with large numbers of commits.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support 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?

Closes #27345 (moved)