Preserves formatted_email structure

This builds on the work and comments made in https://github.com/alphagov/email-alert-service/pull/24.

Discussion copied from there:

I would prefer to add the "public_updated_at" to EmailAlert#format_for_email_api and pass that structure around (both to the worker and to the lock handler).

Preserving the whole structure will avoid assign additional local variables in the worker and leave both classes open for extension. We also risk introducing shotgun surgery otherwise, since any changes in format email will result in changes to instantiating the lock handler in the worker.

It will also prevent churn caused by the introduction of the "formatted" key, that does not seem to bring any benefits.

We should also look to be consistent in the way hashes are accessed - happy to move to fetch but consistently.