Refactor fact check config

Configure fact check sending and receiving settings from a single YAML config file, rather than a Ruby file that configures mail settings and some hard-coded logic in the `govuk_content_models` gem.

This lets us have custom fact check settings for preview and production, so we can test fact check more thoroughly; we can also migrate away from a fact check address on the old `alphagov.co.uk` domain.

This shouldn’t be merged before the corresponding pull request in `alphagov-deployment`.