Move the logic of EPUB creation to the corresponding microservice
As we have already the EPUBChecker microservice which checks the validity of a created EPUB from Ketida, it makes sense to move the entire creation of the EPUB to that service too. The benefit of that will be that we will offload work from the main server of Ketida. This will provide better scoping/sandboxing, scaling and also will allow us to move towards the idea of a server which its main task will be to just server data. This, will increase server's performance and reliability as heavy task (async etc) will be delegated to specific satellite microservices.