Refactor user popover to use `GlAvatarLabeled`

What does this MR do and why?

Related to #363510 (closed)

Refactors the user popover shown when hovering over a user to use GlAvatarLabeled instead of a custom implementation.

Screenshots or screen recordings

State	Before	After
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Loaded

How to set up and validate locally

1. Navigate to an issue or MR
2. Hover over a user's name
3. If you want to update the user's profile you can do this by finding the user in /admin/users and then clicking Impersonate. You can them update their profile in /-/profile

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.