Add/use field for currentChatDiscussionId?

[vivtsai]

This field can be part of publicStageData and auto-update if a participants marks "ready to end" and all other current participants are also ready to end.

Right now, we calculate this based on whether/not all participants previously marked "ready to end"—however, this potentially gets messed up if a new participant joins the experiment. (Can do some digging to confirm that using a field would be an improvement.)