enqueue: Refactor 'evaluateCommand' out of 'run'

This makes it a little clearer that the Command interpreter should
produce a list of emails, and not have to worry about actually publishing
these on the message queue.
Use 'noreply@musicbrainz', not 'no-reply@musicbrainz'
Use explicit import lists or qualified imports
mailer: Attempt to refactor 'emailConsumer' to be the sum of smaller programs

'emailConsumer' is now a few IO actions, followed by a call into 'maybe' on JSON
deserialization, and then an 'EitherT' action to try and send the email,
handling a failure by dispatching to the unroutable queue.