Reduce size of assignee and reviewer rows in sidebar

What does this MR do and why?

Reduces the size of the assignees and reviewers in the sidebar to be 24px, this is done by changing the size of the avatar and removing the users username.

With the removal of the users username this also adds the user popover to the assignee and reviewer rows, this popover will show the users username.

Screenshots or screen recordings

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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Related to #356703 (closed)