K-Fêt' groups edits don't remove non-kfet app permissions.

Fixes #168.

kfet/forms.py

@@ -167,10 +167,22 @@ class GroupForm(forms.ModelForm):
         name = self.cleaned_data['name']
         return 'K-Fêt %s' % name
 
+    def clean_permissions(self):
+        kfet_perms = self.cleaned_data['permissions']
+        # TODO: With Django >=1.11, the QuerySet method 'difference' can be used.
+        # other_groups = self.instance.permissions.difference(
+        #     self.fields['permissions'].queryset
+        # )
+        other_perms = self.instance.permissions.exclude(
+            pk__in=[p.pk for p in self.fields['permissions'].queryset],
+        )
+        return list(kfet_perms) + list(other_perms)
+
     class Meta:
-        model  = Group
+        model = Group
         fields = ['name', 'permissions']
 
+
 class AccountNegativeForm(forms.ModelForm):
     class Meta:
         model  = AccountNegative