Use GlAvatar in design note

What does this MR do and why?

Related to #355864 (closed).

Replaces UserAvatarLink with GlAvatar.

Size=40 doesn't exist in GlAvatar, so I've had to make the avatar smaller, to the default size 32.

Screenshots or screen recordings

before	after

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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