Resolve "Issue Sidebar : Don't render participants in collapsed state"

What does this MR do?

Defer/Lazy load participant avatars so they don't show when the sidebar is collapsed or when the 'more participants' area is not toggled visible.

When the sidebar or the 'more participants' area is toggled, we explicitly call the lazy loader loadCheck method to force a load of the newly visible imgs.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

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 #34366 (closed)