Add clean_value method to ProductAttributeAdminForm

Use the form field validators provided by Django.
Fix the test_clean_attribute test of ProductAttributeAdminForm
Restructure for abstraction.

Create base abstract models that hold the most important information,
any custom modification will be done through inheritance.

Create mixins that can be used with some of the base models.
Remove unused files.
Add the missing files.
Explicit * imports.
Fix the use of __all__.
Add BaseCategoryForm.

A ModelForm that can be used only with model inheriting from BaseCategory.
Remove the plethora appls from tests settings.

Not needed anymore.