First Deployment using Releases

Trying a new approach for triggering deployments using releases, rather than automatically on push to a branch.

This change splits the workflows into two files:

• checks.yml, for running tests. Normally, this wouldn't run on the main branch.
• deploy.yml, for deploying. Triggered on releases. Uses the production environment, which requires me (@grahamhoyes) to approve and can only be run from the environments-and-releases branch (at the moment).