Avatar: Refactor img to use GlAvatar

What does this MR do and why?

This is a follow-up to the reverted MR Avatar: Refactor to use gl-avatar (!163899 - merged)

We'll split up the changes into smaller pieces to not introduce any regressions. This MR only alters the following:

• Refactors <img> to use GlAvatar (Vue only)
• Removes .avatar-cell CSS class in favor of the utility class .gl-self-start

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How to set up and validate locally

1. Board list header: http://gdk.test:3000/groups/flightjs/-/boards/3?assignee_username=arie.wyman
2. Assignee avatar: http://gdk.test:3000/flightjs/Flight/-/merge_requests/4
3. Reviewer avatar: http://gdk.test:3000/flightjs/Flight/-/merge_requests/4
4. MR widget author: (Merged MR, e.g. http://gdk.test:3000/flightjs/Flight/-/merge_requests/1)
5. Commits: http://gdk.test:3000/flightjs/Flight/-/commits/master?ref_type=heads

Related to #474921 (closed)