fix: delete perm on user/group/team deletion

This is done manually using signal because we do not use a foreign key
so we do not have automatically cascade deletion.

permissions/models.py

@@ -1,6 +1,7 @@
 from django.db import models
 from django.db.models import Q
 from django.utils.encoding import python_2_unicode_compatible
+from django import VERSION
 
 from tracker.models import Project
 from accounts.models import *
@@ -68,7 +69,6 @@ class PermissionModel(models.Model):
     @property
     def type(self):
         return self.get_grantee_type_display()
-#        return dict(self.GRANTEE_TYPE)[self.grantee_type]
 
     @property
     def name(self):
@@ -142,3 +142,7 @@ class ProjectPermission(PermissionModel):
     def __str__(self):
         return self.grantee.__str__() + "'s permissions on " \
             + self.project.name + " project"
+
+
+if VERSION < (1, 7):
+    import tracker.signals