[Tests] Write automated Postman tests for the API

Test the most important API calls:

- get apps :arrow_right: check that all info for apps are retrieved
- add app :arrow_right: check that all info for the app are saved
- edit app :arrow_right: check that all info for the app are edited

Useful links:
- https://blog.getpostman.com/2014/03/07/writing-automated-tests-for-apis-using-postman/
- https://blog.getpostman.com/2014/04/17/how-to-write-automated-tests-for-apis-with-postman-part-2/
- https://blog.getpostman.com/2014/05/09/writing-automated-tests-with-postman-part-3/

Let us know if you would like to use a different testing framework :smile:

Best practices for reporting new issues:
- add the [Bug] prefix in the title
- clearly mention the steps to reproduce the bug and the expected behaviour