app.js: prevent mutation of global config

[remi-dupre]

I feel like the best way to fix this indefinitely would be to clone the global config, but it doesn't seem like this is something idiomatic in Javascript: https://attacomsian.com/blog/javascript-clone-objects (describes a solution based on a library, and the other one consists in serializing then de-serializing JSON ... lol nope).

So I guess we will have to be careful never mutating the global configuration.

EDIT: maybe we'd want to use the no-mutation lint from https://github.com/jhusain/eslint-plugin-immutable ? It seems unacceptable to me that eslint accepts this by default:

const obj = { a: 3 };
obj.a = 0;