Epic: Use GraphQL to query participants
participants from GraphQL in the sidebar.
Epics load participants on the initial page-load rather than in an asynchronous call, as issues and MRs do. For epics with a lot of participants this can cause long page-load times.
Related to #119333 (comment 263671170)
Use an AJAX call for sidebar extras instead.
We're in the process of moving sidebar loading to GraphQL, so it may be better to expose this on the backend through it.