Unify all mail templates into a separate module

Mail templates are theme specific.
In order to create an email current store theme is resolved then appropriate template is found and an finalised email is rendered and stored as mail entity.
We have two web apps that perform this: storefront and server (with a third one coming on the way REST API)

We need to extract mail templates into a separate project and then use mvn war plugin to include these resources. This way we can avoid duplication and unify approach on how mail templates are developed.