[Releng] Switch releng to GitHub actions

[Guite]

Use workflows for GitHub actions to perform tests, analysis and build releases.

This entirely replaces external tools (e.g. StyleCI, Travis) as well as our Jenkins instance and moves continuous integration into this repository.

I am already using it for smaller repositories and collecting experience.

Further information: about actions, docs

• Replace release building / separate Jenkins instance
• Update CoreManager module accordingly (done in zikula/CoreManagerModule@c506df6)
• Review and decide about next steps (see also [Releng] Evaluate tools for test coverage #3665)

[Guite]

possibility: php-cs-fixer with symfony preset instead of StyleCI