Decouple `actix-web` handlers from app services

[josecelano]

The app services are tightly coupled to actix-web handler. That makes it more challenging:

• To migrate to a new framework is needed. In fact, we were planning to do it.
• To test the services because:
  • You need to start the whole app
  • Assertions have to deal with the framework requests and responses
  • It's slower
  • You cannot easily mock service dependencies

The plan is just to extract the handler bodies into independent app services.