Fix username and avatar display on the review leaderboard

[coderabbitai[bot]]

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Background

This is a follow-up to issue #3319.

The first part of the original issue has been completed - the pull request leaderboard now shows contributors who don't have BLT accounts.

Requirements

Fix the username and avatar display on the code review leaderboard:

• Ensure reviewers' usernames are displayed correctly
• Fix avatar display for reviewers without BLT accounts
• Ensure the leaderboard can show GitHub reviewers who don't have BLT accounts

Current implementation

The review leaderboard section exists in leaderboard.html but may have issues with displaying the correct usernames and avatars.

Related PRs

• PR #3763 implemented the fix for the PR leaderboard part of this issue

Requested by @DonnieBLT