Load participants async

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3231

• Load issuable sidebar participants async
• Load notifications subscribe/block participants async (relies on participants)

Collapsed	Expanded

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

• Issue boards doesn't have participants but does have subscription state although there is an existing backend issue which is causing a bug there, #39167 (closed) - I'll create a separate MR to switch over issue boards to this new Vue sidebar piece once that BE issue is fixed and remove the existing JS holding that together.

Why was this MR needed?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• 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 #23206 (closed)