Fix (all) the lint errors & warnings

About the issue

At the moment there are about more than 3000 lint errors present in the code. Gradually all of those should be taken care of.

Right now the fixes are done on the branch lint_fixing

The current issues can be shown with the terminal command npx eslint . --ext .ts optionally with the --fix flag.

Commits should separate the task either a) file-based if there is a substantial amount of lint errors within a single file present or b) folder-based if not. Commit message should first of all contain the complete file name. If there are errors or warnings left that couldn't be resolved, please add a note to the message.