Consider switching from jest-fetch-mock to fetch-mock

[michielbdejong]

We both ran into the issue of order in which you mock URLs which is super annoying. This issue was reported here, and as no real solution seems to exist, people suggested to use fetch-mock instead.

Now, it also seems empty response bodies are not supported by jest-fetch-mock so I suggest we give fetch-mock a try?

[angelo-v]

I am fine with that