Symfony 4 upgrade

We completely refactored a project for upgrading from Symfony 3.x to the latest 4.x version.
The Symfony 4 versions are much faster in response times. Also the architecture is more modern.

Due to the revised architecture, we had to refactor large parts of the application:

• The way the application is configured is revised
• We now use a much simpler and developer-friendly code structure
• We changed many services to 'auto wired' services

Developers can now use the latest stable Symfony technology.