Allow to use dynamic content for generating emails

As a marketing user I would like to be able to modify content of the mail.

Tech:
We need to reuse our dynamic content capabilities in order to try resolve mail body from shop content and only if it is not found we should use the theme mail templates.
This should be accomplished by injecting contentService into mailComposer and use dynamic content API to inject message details. We can also reuse content custom image attributes in order to set header images for example.