Externalize seneca-email into a separate microservice

[Wardormeur]

The goal is to unify processes which are duplicated into one single place, cp-email-service Multiple reasons :

• Templates and translations are pulled twice (users, dojos) hence a potentially better deployment time once reunited
• Configurations should be the same (but is not atm, gmail is used in cp-users while it's sendgrid on cp-dojos (which is a mistake) ) which should help code maintenance
• the worker for seneca-kue would be the same for cp-dojos & cp-users Related to https://github.com/CoderDojo/community-platform/issues/1112

[Wardormeur]

This as been done with the deployment of https://github.com/CoderDojo/cp-email-service. Closing!