Fix broken url on group avatar

What does this MR do?

Fix broken url on group avatar in mention autocomplete in editor when GitLab installed under relative URL.

Why was this MR needed?

Not shown group avatar when GitLab installed under relative URL.

Screenshots (if relevant)

before

after

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

Closes #26452 (closed)