Create API to send arbitrary email messages

Parent ticket #34 (closed)

At some stages, we need to allow users to write and send messages to other users over email (particularly from admin to submitters and reviewers).

We need to be able to send recipients, a subject and message to an API and have those sent.

What the API should use:

• See @pubsweet/component-send-email https://gitlab.coko.foundation/pubsweet/pubsweet/tree/master/packages/components/SendEmail-server

• See example of SMTP mailer config from elife https://github.com/elifesciences/elife-xpub/blob/develop/config/mailer.js