Investigate Terraform provider for Netlify

It seems the [Terraform provider for Netlify](https://registry.terraform.io/providers/netlify/netlify/latest/docs) may have improved since the last time we tried to use it. Adopting this to manage our Netlify deploys would bring some advantages, including the ability to stop maintaining production environment variables in .env files in the repository (which may lead to clones of the deployment accidentally using those values and polluting the main deploy's data sources).