Resolve "Sonarcloud for Static Analysis"

Acceptance criteria

General

• CI runs without problems
• Migration scripts exist and work for existing data
• Code does contain license headers

Code quality

• Code does not contain variables with abbreviated names
• Code does not contain TODOs, unused functions or console outputs
• Code has been inspected and deemed to have decent quality
• If this is a bug fix, a test covering the bug is present
• All javascript methods have been commented using Doc-comments(/** */)
• Tests for added backend code are present

Documentation

• If necessary, the changes are documented in the changelog
• Newly used libraries have been added to the "about.tmpl"
• If changes to the documentation are necessary (frontend changed or new frontend features were added), an issue for said changes has been created

Closes #727 (closed)