Clean up IndexController and related code

What does this MR do?

Just some cleanups in and around the IndexController. As a bonus, I got rid of some global variables!

Please review commit by commit, it will make more sense.

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

Very confident

How to test

Just low level refactoring, nothing you can really 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
• 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.