Foundation for porting controllers to symfony (iteration 1)

What does this MR do?

This is a basic foundation for porting old controllers to new Symfony controllers. The process is documented in the dev docs (file php-controller-migration.md, linked in php-controller.md).

Once this is merged, I will follow up with a relatively simple controller port (to serve as an example MR that I'll link in the devdocs then).

How confident are you it won't break things if deployed?

Very confident, besides adding so far unused code, I only extracted logic 1:1 from IndexController to put it in an EventSubscriber.

How to test

low level refactoring, so nothing specific to test

Checklist

• added a test, or explain why one is not needed/possible...
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• add a short text that can be used in the release notes
• Once your MR has been merged, you are responsible to create a testing issue in Beta Testing Repo:
  • Consider writing a detailed description in German.
  • Describe in a few sentences, what should be tested from a user perspective.
  • Also mention different settings (e.g. different browsers, roles, ...). how this change can be tested.
  • Be aware, that also non technical people should understand.