Add is_checked_in into joined event api

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

data/event.go

@@ -34,6 +34,7 @@ type EventIndexDoc struct {
 	Thumbnail    string    `json:"thumbnail"`
 	EnableKYC    bool      `json:"enable_kyc"`
 	IsJoined     bool      `json:"is_joined"`
+	IsCheckedIn  bool      `json:"is_checked_in"`
 	JoinCount    int64     `json:"join_count"`
 	CheckinCount int64     `json:"checkin_count"`
 }