Since Travis CI was transformed into a proprietary service we didn't have a CI pipeline. This heavily refactors and simplifies the CI pipeline and uses GitHub Actions to deploy new Pico releases. You can use Pico's Makefile to build new releases locally, too. Pico's build script no longer depends on any external resources (like our `picocms/ci-tools` collection). However, this isn't true for other release deployment steps, like updating phpDoc class docs, version badge, cloc stats, etc., even though the CI scripts are mostly self-containing now.
