Some model forms need refactoring

[bronger]

The model forms that use choices_of_content_types need to be refactored. This work has begun in the branch “choices_of_content_types”. Unfortunately, this is not easy, so I had to stop this due to time constraints. Things need to be considered:

• How can the code be simplified, now that the choices tuples have a ContentType instance in their second field?
• Is it necessary to re-define many of the fields?
• Is it necessary to save M2M relations to ContentType separately after the ModelForm.save()?
• Is __all__ necessary?

Of course, it needs to be tested:

• Whether the ordering really is according to the name of the ContentType.
• Whether everything is still properly saved.
• Whether empty MultipleChoiceFields are properly displayed and processed.