MM-65575: Fix server panic when bot posts trigger persistent notifications (#34174) (#34778)

* reproduce panic with test

* allow bots in the profile map

* explicitly prevent sending notifications to bots

* persistent notifications: handle senders not in the channel

server/public/model/notification.go

@@ -40,4 +40,5 @@ const (
 	NotificationReasonTooManyUsersInChannel              NotificationReason = "too_many_users_in_channel"
 	NotificationReasonResolvePersistentNotificationError NotificationReason = "resolve_persistent_notification_error"
 	NotificationReasonMissingThreadMembership            NotificationReason = "missing_thread_membership"
+	NotificationReasonRecipientIsBot                     NotificationReason = "recipient_is_bot"
 )