[CI] Use BrowserStack for Selenium tests
We got a sponsorhip from BrowserStack, a (commercial) multi-browser testing provider. Through that, we get access to a virtually unlimited set of real devices and thus can run Selenium tests against things like Edge on Windows, Mobile Safari on real iPhones, and browsers on real Android devices.
Note: BrowserStack is not free software, so the test suites of apps must never depend on it. We can, however, inject settings to use it in our CI files, which are only relevant when used here on EduGit. A service like BrowserStack that provides hundreds of real mobile devices for testing cannot operate non-commercially (however, the integration ocde could still be free software).