Code quality improvements

• Add linting to pipeline (including a configuration adapted to our project)
• Improve code quality by
  • adding more comments (inline and for methods and classes)
  • removing of code smells and style issues (e.g., unused variables and arguments)
  • simplifying statements
  • cleaning up imports
  • removing duplicated code fragments through the usage of mixins
  • move code fragments to better places
• Improve API
• Introduce auto documentation building and serving as part of the deployed app
• Remove empty admin, model and test files
• Add additional tests and enhance test framework
• Reduce CI load (combine checks for code and migration into one job, since every job requires the full setup of the working environment)
• Update translations to changed line numbers