KFetGroup model
- Provides a distinction from non-kfet Groups.
- Convert code appropriately.
- Initially filled from Groups containing K-Fêt (this was the previous
distinction) in the kfetauth.0002 migration.
Permission proxy model (kfetauth app)
- Proxy of the django.contrib.auth Permission model.
- Adds the 'kfet' manager which returns only kfet-related permissions.
KeepUnselectableModelFormMixin
- Helps to keep the unselectable items of many-to-many field for
ModelForm.
- 'kfetauth' forms (related to KFetGroup) use this mixin.
Using KFetGroup allows to simplify the 'kfet/account_group_form.html' template.
A bug is also fixed in 'kfet/form_field_snippet.html', which could lead to
prevent field displays if they used CheckboxSelectMultiple widget.
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