Resolve "Private Avatars are not CDN compatible"

What does this MR do?

It adapts our avatar logic for project and groups to take into consideration if they are private or not and based on that it will use the currently set CDN Url or not for displaying. Part of this was the adaption of group_icon method to deliver an actual image and not an url anymore so it is inline how project_icon works.

Why was this MR needed?

So private project or group avatars also work with a set CDN

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #38245 (closed)

Closes https://gitlab.com/gitlab-com/support-forum/issues/2476