Reworked https://github.com/grafana/oncall/pull/5047. Main update is the
switch from FK to a [M2M
relation](https://docs.google.com/document/d/1HeulqxoFShSHtInQrZNJLL5MDlHPNT50rVGaK3zZWvw/edit?disco=AAABVLjV4W8)
(which doesn't really change the original/intended behavior, besides not
needing to alter the alert group table, and it is a bit more flexible;
the extra table shouldn't introduce issues because this is used only for
tracking purposes and the information needed in the log record is
already there).
Avoid a db migration involving alert group table:
```
--
-- Create model RelatedIncident
--
CREATE TABLE `alerts_relatedincident` (`id` bigint AUTO_INCREMENT NOT NULL PRIMARY KEY, `incident_id` varchar(50) NOT NULL, `created_at` datetime(6) NOT NULL, `is_active` bool NOT NULL, `channel_filter_id` bigint NULL, `organization_id` bigint NOT NULL);
CREATE TABLE `alerts_relatedincident_attached_alert_groups` (`id` bigint AUTO_INCREMENT NOT NULL PRIMARY KEY, `relatedincident_id` bigint NOT NULL, `alertgroup_id` bigint NOT NULL);
ALTER TABLE `alerts_relatedincident` ADD CONSTRAINT `alerts_relatedincident_organization_id_incident_id_d7fc9a4f_uniq` UNIQUE (`organization_id`, `incident_id`);
ALTER TABLE `alerts_relatedincident` ADD CONSTRAINT `alerts_relatedincide_channel_filter_id_9556c836_fk_alerts_ch` FOREIGN KEY (`channel_filter_id`) REFERENCES `alerts_channelfilter` (`id`);
ALTER TABLE `alerts_relatedincident` ADD CONSTRAINT `alerts_relatedincide_organization_id_74ed6bed_fk_user_mana` FOREIGN KEY (`organization_id`) REFERENCES `user_management_organization` (`id`);
CREATE INDEX `alerts_relatedincident_incident_id_8356a799` ON `alerts_relatedincident` (`incident_id`);
ALTER TABLE `alerts_relatedincident_attached_alert_groups` ADD CONSTRAINT `alerts_relatedincident_a_relatedincident_id_alert_3d683baa_uniq` UNIQUE (`relatedincident_id`, `alertgroup_id`);
ALTER TABLE `alerts_relatedincident_attached_alert_groups` ADD CONSTRAINT `alerts_relatedincide_relatedincident_id_3e5e7a23_fk_alerts_re` FOREIGN KEY (`relatedincident_id`) REFERENCES `alerts_relatedincident` (`id`);
ALTER TABLE `alerts_relatedincident_attached_alert_groups` ADD CONSTRAINT `alerts_relatedincide_alertgroup_id_0125deca_fk_alerts_al` FOREIGN KEY (`alertgroup_id`) REFERENCES `alerts_alertgroup` (`id`);
```
fa815b7ecd