This PR aimes to [fix this issue](https://github.com/coder/internal/issues/448) - The main idea is to remove greetings from templates stored in the DB - and instead push it into the template for require methods - for now SMTP.
Relates to https://github.com/coder/coder/issues/16463 Adds a CLI command, and API endpoint, to trigger a test notification for administrators of a deployment.