Create database tables for escalation policies and rules

What does this MR do?

This adds two tables needed for the Escalation Policies epic.

Model: IncidentManagement::EscalationPolicy

Column	Required	Type
id	true	Integer
project_id	true	Integer
name	true	text
description	false	text

Validations/constraints:

• project, name should be present
• name length should be under 72 chars
• description length should be under 160 chars
• Must have at least one EscalationRule
• Unique constraint: name should be unique within the project.

Table: incident_management_escalation_rules

Model: IncidentManagement::EscalationRule

Column	Required	Type	Description
id	true	Integer
policy_id	true	Integer	IncidentManagement::EscalationPolicy to which the escalation rule belongs
status	true	Integer	One of AlertManagement::Alert::STATUSES values for either :acknowlegded or :resolved
elapsed_time	true	Integer	Time in minutes from the creation of an alert at which this rule should be enacted
schedule_id	true	Integer	IncidentManagement::OncallSchedule which should be notified in escalation rule

Validations/constraints:

• project, status, elapsed_time, schedule should all be present
• schedule_id should belong to the project of escalation policy
• status should be one of AlertManagement::Alert::STATUSES values for either :acknowlegded or :resolved (one of [1, 2])
• elapsed_time should be an integer >= 0 & <= 24.hours
• Unique constraint: Combo of policy_id, status, elapsed_time, schedule_id should be unique. This is all the attributes. Is there a simple way to do this? Should we allow duplicate rules?

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #323138 (closed)