102 refactor to typescript

[hoelger]

Todo: run prettier

EDIT:

Why can't I simply reply on this comment? Hmpf...

I ran npm run-script lint-fix and npm run-script postlint-fix, the first one also changed some styling. But when I now change some indentation in /app then those commands don't revert these changes. What? My best proposal for this is:

• Extract this in some follow-up issue where:
• The procedure/commands are documented in the README and
• Once you set up everything correctly (e.g. with pre-commit) you're acutally forced to comply to the styling rules

[hoelger]

I can change the target but I can't change the source of a PR? hmpf... Oh how I prefer gitlab over github! <3