Add slash command to page incident

What does this MR do and why?

Implements #330282 (closed)

Adds /page <escalation_policy_name> quick action command to page an incident with an escalation policy

Screenshots or screen recordings

page_command_demo

Query plan and execution

I ran this query on a sample project

https://console.postgres.ai/shared/4e2416e0-290d-41a4-83dd-6fd460d28846

How to set up and validate locally

1. Add an escalation policy via UI as mentioned in !61815 (merged)

2. Create a new issue of type incident.

3. Type /page <escalation policy name from step 1> as a comment.

4. Check flash messages and system notes when escalation is triggered.

5. Check counter is increased for events in redis for tracking using the following line in rails console.

Gitlab::UsageDataCounters::HLLRedisCounter.unique_events(event_names: 'i_quickactions_page',
   start_date: Date.current.beginning_of_week, end_date: Date.current.next_week)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #330282 (closed)