Add optional CloudFront distribution for app server

[tobiasmcnulty]

For CMS sites, it can be helpful to have a CloudFront distribution in front of the application server. This PR creates a new CloudFront distribution with a custom origin pointing to the load balancer associated with the EB, ECS, Dokku, or EC2 instance(s) in the stack.

Update 6/27/23: The CloudFront distribution was pulled out to its own stack so it can be more easily deployed regardless of how the application is served (including support for multiple apps in a single main stack).