Tous les `on_delete` étaient mis à PROTECT, ce qui faisait qu'on ne
pouvait rien supprimer... On les met à CASCADE pour tous les modèles
secondaires (`AccountNegative`, `CheckoutStatement`, `InventoryArticle`,
`SupplierArticle`, `Order` et `OrderArticle`) et pour les inventaires
créés à partir d'une commande.
Pour les modèles qui demandent une validation, et pour les `Operation`s,
on met à NULL le compte ou l'article associé (cela ne change pas le
total d'une opération, qui est la partie importante à garder).
If content type for old kfet.GlobalPermissions exists:
- custom permissions related to this content type are updated to new content types,
- then we can safely remove this content type.
- kfet_config gives "reduction_cof" as editable instead of
"subvention_cof"
- this last one can still be accessed via kfet_config (computed from
new "reduction_cof"
- add units to numeric values of kfet_config form
- add verbose names to ArticleCategory fields
- add button to view categories list from articles list
- fix article_update template in form validation
- improve interface for articlecategory_update
- revert vanished urls (happened in merge with master...)
Old configuration(/settings), based on Settings model, system is
deleted: SettingsForm, Settings.
New system use `django-djconfig` module.
- `kfet.config` module provides `kfet_config` to access configuration concerning
kfet app.
- Views, forms, models, etc now use this object to retrieve conf values.
- Views no longer add config values to context, instead templates use
`kfet_config` provided by a new context_processor.
- Enhance list and update views of settings.
- Fix: settings can directly be used without having to visit a specific
page...
Misc
- Delete some py2/3 imports
- Delete unused imports in kfet.models and kfet.views
- Some PEP8 compliance
