implement notification group (#450)

* implement notification group

* some fixes

* fix sql

* add listNotification

* retrieve notification from map

* create notification_group_notification if non-exist

* NotificationIDToGroup -> NotificationIDToGroups

* clean

model/notification_group_api.go

@@ -0,0 +1,11 @@
+package model
+
+type NotificationGroupForm struct {
+	Name          string   `json:"name"`
+	Notifications []uint64 `json:"notifications"`
+}
+
+type NotificationGroupResponseItem struct {
+	Group         NotificationGroup `json:"group"`
+	Notifications []uint64          `json:"notifications"`
+}