Move the field types into a separate file

[llybin]

Hey. At the moment, the field types are listed inside the model.

For this reason, it’s not possible to use the field type constants in settings because of the error:"django.core.exceptions.AppRegistryNotReady: Apps aren't loaded yet."

I suggest moving the constants into a separate file and updating the documentation on how to use them.

The reason for this proposal also is a missed error in one field in my project, where instead of string it was mistakenly written as str

[fabiocaccamo]

@llybin do you want to work on a PR for this?

[llybin]

I created a task on our board, within the priorities.