Migrate Fronts-based Email Newsletters To Email-Rendering

[JamieB-gu]

They're currently rendered by frontend, and have limited configuration options in the Newsletters Tool. This means that manual code changes are often required. For example, to add or update a banner image, as in #27339, #27422 and #27440

They should be migrated to the email-rendering project, and made configurable by Editorial via the Newsletters Tool. The work to do this has been started in https://github.com/guardian/email-rendering/pull/394