Add utility method `get_updated_model()` to `ModelForm`

[Javier Candeira]

Add utility method get_updated_model() to ModelForm and, additionally, add utility method get_updated_models() to FormSet.

Rationale:

While doing the djangogirls tutorial, I noticed that ModelForm.save(commit=False) is given to newcomers as a reasonable way to acquire a form's populated model. This is an antipattern for several reasons:

• It's counterintuitive. To a newcomer, it's the same as save(save=no).
• It's a leaky abstraction. Understanding it requires understanding that the save method does two things: a) prepare the model, and b) save it to the database.
• It doesn't name its effect or return well. It returns a model, not a form.

All these problems are addressed in the current patch. Changes:

• Implement ModelForm.get_updated_model()
• Implement FormSet.get_updated_models()
• Refactor models.py minimally for the above to work.
• Tests not added for get_updated_model() nor get_updated_models() because the changes are small enough to be considered as a refactoring.
• Both the tests and contrib.auth have been modified: any call to save(commit=False) for the purpose of obtaining a populated model has been replaced by get_updated_model(). Tests still pass, I'm confident it's a successful refactor.
• docs/customizing.txt has also been modified to showcase the new method.

Notes:

Uses of ModelForm.save(commit=False) in the codebase have been left alone wherever it was used for its side effects and not for its return value.

The PR at ​https://github.com/DjangoGirls/tutorial/pull/450 depends on the present patch. Its changeset there explains the rationale for this addition better than anything else.