Issue #2214057 by g3r4: Remove obsolete LanguageManagerInterface::init() method.
Issue #1671498 by g3r4, Jorrit: Fixed Docs of template_preprocess_html() refer to wrong hook.
Issue #2077979 by cilefen, plopesc: Modernize filter.module forms: use $this->t().
Issue #2287223 by tim.plunkett: Use KernelTestBase for config schema tests where possible.