Disambiguation in kfet's permission handling

In some places we used to refer to permissions based on their codename
only (the part after the dot "." in the following examples) which can be
ambiguous. Typically, we might define permissions like "bds.is_team" or
"cof.is_team" in the near future ;)

kfet/tests/test_statistic.py

@@ -18,7 +18,7 @@ class TestStats(TestCase):
         user.set_password("foobar")
         user.save()
         Account.objects.create(trigramme="FOO", cofprofile=user.profile)
-        perm = Permission.objects.get(codename="is_team")
+        perm = Permission.objects.get_by_natural_key("is_team", "kfet", "account")
         user.user_permissions.add(perm)
 
         user2 = User.objects.create(username="Barfoo")