Add slash command for escalating an incident

Release notes

Managing incidents can be very time sensitive. Having a quick action to start escalations for an incident will save users time and make them more efficient.

Motivation

Managing an incident can be very time sensitive. Having a quick action to start escalations for an incident will save users time.

This issue as a lot of overlap with the backend work from #330284 (closed). In order to de-duplicate work, these two items should be completed in series (rather than in parallel).

Scope

• Add new slash command /page <escalation-policy>
  • Escalation policy name should be a required argument
  • Should only be available to developer+
  • Should only be available on manually created incidents (which do not have an associated alert)
• Calling slash command should:
  • Create a IncidentManagement::IssuableEscalation to begin escalations
  • Immediately execute any EscalationRules which are set to occur after 0 minutes
  • Switch the status of the incident to Triggered if called with a different escalation policy than the one already being used