Add tables/models for IssuableEscalationStatus & IssuableEscalations

Motivation

These tables and models represent the data structures required to escalate manually created incidents according to escalation policies. The primary goal of this issue is to unblock the next pieces of work so that they can be taken in parallel.

These tables will be used with the logic built in #330281 (closed).

Scope

• Add 2 new tables.
• Add 2 new models & validations.
• Add relevant associations to existing models.

Implementation Plan

⚠ This approach assumes we can't add new fields to issues directly, as has been the case with severities & SLAs.

Table: incident_management_issuable_escalation_statuses

Model: IncidentManagement::IssuableEscalationStatus

Column	Required	Type	Description
id	true	Integer	ID of the object
issue_id	true	Integer	Incident which has an escalation status
status	true	Integer	One of AlertManagement::Alert::STATUSES

Validations/constraints:

• issue, status should both be present
• status should be in AlertManagement::Alert::STATUSES
• Unique constraint: issue_id should be unique

Table: incident_management_issuable_escalations

Model: IncidentManagement::IssuableEscalations

Column	Required	Type	Description
id	true	Integer	ID of the escalation
policy_id	true	Integer	Escalation Policy to which the escalation corresponds
issue_id	true	Integer	ID of the escalate-able IssuableEscalationStatus
created_at	true	datetime_with_zone	Creation time of the escalation (AKA - time at which the escalation was "triggered")
updated_at	true	datetime_with_zone	Update time of the escalation (AKA - time at which notifications were last sent out)

Validations/constraints:

• issue, policy should both be present
• Unique constraint: issue_id should be unique
• Test/confirm that adding IssuableEscalation does not break the Protect stage's implementation for Alerts. Additional context. Docs.