Convert the functional tests to Docker-based tests

Move the JUnit4 Selenium tests of application-blog-test to the JUnit5 Docker-based test framework (@UITest), so that new functional tests can be written with the current framework.

• Keep the current minimal XWiki version unless a needed feature of the test framework requires a newer one, in which case use the oldest version providing it.
• Convert the existing test classes first, then remove the old JUnit4 setup.