Move non business logics code out of services folder

[edisonzsq]

In general, you want to categorize your code so that other developers can easily find the right code in the right place. You can consider moving services/service-logger to a helpers folder. The path may look like groupproject2-book/helpers/logger.helper.js or it can be under groupproject2-book/configs/logger.config.js.

The principle is to categorize your code for easy finding.