Add next_id method to Variant model.
Introduce Dialog and Attempt models.
Add load_dialog_data manage.py command.
Save slides objects with load_dialog_data command.
Move Variant model to django.models.Model.

Deinstall mptt package.
Save variant object with load_dialog_data command.
Add model validation in json files.
Add number field to Slide model.
Separate load_dialog_data command in several functions.