User avatar in commit message trailers is not visible

Summary

A user avatar (at least without Gravatar) in commit message trailers is not visible.

Steps to reproduce

1. Prepare a user account with its email address of "<johnsmith@example.com" without Gravatar setting.
2. Create a commit with its message of "\nSigned-off-by: John Smith johnsmith@example.com" on Web UI.
3. Open the commit made as above on the project commits and open its collapsed commit trailer to see the avatar image.

Example Project

• the most top commit of https://gitlab.com/gitlab-com/www-gitlab-com/commits/02d4d7ec29668f6aeeeffde2ab28506c632438be

What is the current bug behavior?

The avatar image on "Signed-off-by" line (the red arrow) is not visible due to its 404 status.

The small avatar image is https://gitlab.com/gitlab-com/www-gitlab-com/uploads/-/system/user/avatar/100770/canstock6553672_288.png?width=16

What is the expected correct behavior?

The avatar image on "Signed-off-by" line (the red arrow) should be visible as same as the main avatar (the blue arrow).

The small avatar image should be https://gitlab.com/uploads/-/system/user/avatar/100770/canstock6553672_288.png?width=16

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com (11.7.0-rc4-ee).

Possible fixes

Fix Banzai::Filter::CommitTrailersFilter to fix the avatar image URL and so on...

cf.

The original feature relevant to this bug was made in !17919 (merged).