identities: make BaseIdentitiesStore retrieve its identity class from a class variable

authentic/identities.py

@@ -394,6 +394,7 @@ class BaseIdentitiesStore:
                Field('email', N_('Email'), required = True, size = 30) ]
 
     username_regex = r'[a-zA-Z0-9.-]+$'
+    identity_class = Identity
 
     def create_password(self,for_account='unknown'):
         '''Create a new password for an user contained in this store'''
@@ -486,7 +487,7 @@ class BaseIdentitiesStore:
         return time.time()
 
     def get_identity_class(cls):
-        return Identity
+        return cls.identity_class
     get_identity_class = classmethod(get_identity_class)
 
     def administrators(self):