Terraform: add Mailgun

[0pdd]

The puzzle 1000-66d57f44 from #1000 has to be resolved:

mystamps/infra/terraform/my-stamps.tf

Line 3 in 8e7e2e5

# @todo #1000 Terraform: add Mailgun

The puzzle was created by @php-coder on 23-Mar-20.

[php-coder]

See https://www.terraform.io/docs/providers/mailgun/index.html