Task mentions (#926)

Co-authored-by: kolaente <k@knt.li>
Reviewed-on: https://kolaente.dev/vikunja/api/pulls/926
Co-authored-by: konrad <konrad@kola-entertainments.de>
Co-committed-by: konrad <konrad@kola-entertainments.de>

pkg/notifications/notification.go

@@ -29,6 +29,15 @@ type Notification interface {
 	Name() string
 }
 
+type SubjectID interface {
+	SubjectID() int64
+}
+
+type NotificationWithSubject interface {
+	Notification
+	SubjectID
+}
+
 // Notifiable is an entity which can be notified. Usually a user.
 type Notifiable interface {
 	// Should return the email address this notifiable has.
@@ -82,6 +91,10 @@ func notifyDB(notifiable Notifiable, notification Notification) (err error) {
 		Name:         notification.Name(),
 	}
 
+	if subject, is := notification.(SubjectID); is {
+		dbNotification.SubjectID = subject.SubjectID()
+	}
+
 	_, err = s.Insert(dbNotification)
 	if err != nil {
 		_ = s.Rollback()