configurable BASE_URL

[albsierra]

The BASE_URL constant must be configurable to point towards central repository.
https://github.com/KA226-COVID/java-evaluator/blob/e5a6b8a2efffb79daf3d2752102779fdb0e2a3b2/programming-exercise/CONST.js#L5

Currently, programming-exercise is a npm module. Then, I think that the best form to deal with the value of BASE_URL is as environment variable and we would send his value when we create a new Programming-exercise:

let exerciseObj = new ProgrammingExercise()