Add unit tests for all JavaScript

[muffinresearch]

Any JavaScript that is applied to the site should have unit tests.

• If functionality is broken by changes we will catch the regression.
• It is also important to know when a dependency update breaks existing functionality.

Once we have some tests these can be run by CI on every commit.

[muffinresearch]

We now have jest as a test runner and there's an example of how to configure tests here: https://github.com/mozilla/extension-workshop/blob/05e973ed368075e6d260690ffa1035e3a11aa171/tests/new-tab-links.test.js

This approach can be used for individual files.

For example the newsletter sign-up script would be a good one to start with.