Enable and apply ESLint Jest rules (#3270)

Jest was in the plugin array of the ESLint configuration, but no rules
were enabled. So ESLint hasn't checked any Jest rules yet.

So I activated the recommended Jest rules and added a few more. Then I
fixed the issues (mostly automatically). I have deactivated the rules
"jest/expect-expect" and "jest/no-done-callback" for the time being, as
they would have entailed major changes. I didn't want to make the PR too
big.

I'm not a Jest expert, but the changes so far look good to me. What do
you think of that @khassel? 🙂

tests/unit/classes/translator_spec.js

@@ -197,7 +197,7 @@ describe("Translator", () => {
 				};
 
 				await Translator.load(mmm, file, false);
-				expect(Translator.translations[mmm.name]).toBe(undefined);
+				expect(Translator.translations[mmm.name]).toBeUndefined();
 				expect(Translator.translationsFallback[mmm.name]).toEqual({
 					Hello: "Hallo"
 				});