Support an alert template field to allow for incident customization

What does this MR do?

Adds support for a field gitlab_incident_markdown on prometheus alerts. When users configure self-managed prometheus instances with alert rules and include content in this field, it should be included in the automatically create issue when the alert is triggered - without reformatting. https://gitlab.com/gitlab-org/gitlab-ee/issues/12605

This is to allow for all of the benefits of GFM - including quick actions.

Docs MR on CE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30900

Alert	Resultant Incident
Alert annotations:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• Separation of EE specific content

Performance 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

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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