- Change FIRING trigger for webhooks to be sent after escalation
snapshot has been computed
- Extract users from `notify_to_users_queue` and `notify_schedule` from
escalation snapshot to populate `users_to_be_notified` in webhook
payload
Track all webhook responses data, and allow using this between
alertgroup-related webhooks (e.g. use firing webhook response data when
templating the acknowledge webhook request data).
NOTE: dropping the table is not backwards compatible but the feature is
not enabled (and in any case it would drop log entries only used for
status display)
