Enable CI with GitHub Actions and Sauce Labs

We really should have the tests running automatically when someone submits a pull request. The tests require a browser, ideally multiple current and slightly old ones. This should not be too difficult to set up with Karma and Sauce Labs.

Start by inspecting the existing JavaScript test setup and Karma configuration, then identify how GitHub Actions can invoke it with Sauce Labs browsers. Done means pull requests automatically run the tests across current and slightly old browsers and report their results.

Written by the indexing model from the issue text.