LogMemberLeave
data class LogMemberLeave(val convoId: String, val message: SystemMessageDataMemberLeave, val rev: String)
NOTE: This is under active development and should be considered unstable while this note is here. Event indicating a member voluntarily left a group convo. The member who was removed gets a logLeaveConvo (to leave the convo) but not a logMemberLeave (because they already left, so can't see the system message).