add HealthEventPermission to CloudFormation stack

[wjordan]

Followup to #19125.

This extra AWS::Lambda::Permission resource is required for invoking Slack-forwarding lambda-function from Health Event.

Stack diff:

$ RAILS_ENV=production bundle exec rake stack:validate
AWS CloudFormation Template for Code.org application
Listing changes to existing stack `autoscale-prod`:
Add HealthEventPermission [AWS::Lambda::Permission] 

[wjordan]

Manually updated stack, confirmed that this HealthEvent rule now shows up in the Lambda Management Console for SlackEvent [Triggers] as expected.