Add js tests

What does this MR do?

In preparation for moving more things into webpack style javascript I added some test infrastructure.

They run nice and fast as they just run inside node and use jsdom to simulate a browser environment.

How confident are you it won't break things if deployed?

I moved a few functions out of client/src/script.js into other files, but I think it should be ok...

Links to related issues

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• joined #foodsharing-beta channel at https://slackin.yunity.org
• added an entry to CHANGELOG.md (description, merge request link, username(s))