Render user avatar link using `GlAvatar`

What does this MR do and why?

The feature flag gl_avatar_for_all_user_avatars was introduced in !81437 (merged) to migrate all avatars implementations to GlAvatar. The FF and related code will be deleted when all instances will be checked with FF enabled. This specific MR is targeted to migrating linked avatars to the version using GlAvatar and GlAvatarLink.

This is the supporting MR for the work to be done here.

Screenshots or screen recordings

Where to check	With feature flag DISABLED	With feature flag ENABLED
Go to linked issues section in issue and check related issue assignee's avatar
Go to epics page and check author name in the epic title

How to set up and validate locally

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

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.

Related to #335798 (closed)