Allow `/page` quick action to be used from incident description

What does this MR do and why?

Allow /page quick action to be used from the incident description when creating an incident.

Note that auto-completion of /page does not work yet. See #377513.

Screenshots or screen recordings

Screen_Recording_2022-10-06_at_1.08.54_PM

How to set up and validate locally

1. Create an incident from Monitor > Incidents.
2. Create an escalation policy from Monitor > Escalation Policies.
3. While adding incident description, add /page <policy_name_from_point_2> in the description.
   • ⚠ Auto-completion won't work yet. See #377513.
4. The status must be changed to triggered and the policy must be in effect.

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 #353050 (closed)