Add join count to event api

Signed-off-by: Asai Neko <sugar@sne.moe>

data/event.go

@@ -32,6 +32,7 @@ type EventIndexDoc struct {
 	StartTime    time.Time `json:"start_time"`
 	EndTime      time.Time `json:"end_time"`
 	Thumbnail    string    `json:"thumbnail"`
+	JoinCount    int64     `json:"join_count"`
 	CheckinCount int64     `json:"checkin_count"`
 }