feat(notification): Telegram add support for topics in groups (#894)

* feat(notification): send Telegram messages to a specific topic of a group

* Convert settings.Topic to integer once and reuse it as part of the
telegramSender struct.

* feat(notifications): add migrations for topic

* fix(notifications): find null string

* fix(notifications): form initial values

---------

Co-authored-by: ze0s <ze0s@riseup.net>

internal/domain/notification.go

@@ -41,6 +41,7 @@ type Notification struct {
 	Targets   string           `json:"targets"`
 	Devices   string           `json:"devices"`
 	Priority  int32            `json:"priority"`
+	Topic     string           `json:"topic"`
 	CreatedAt time.Time        `json:"created_at"`
 	UpdatedAt time.Time        `json:"updated_at"`
 }