Fix keys generator

invitations/migrations/0011_auto_20180220_1139.py

 # -*- coding: utf-8 -*-
 # Generated by Django 1.11.4 on 2018-02-20 10:39
 from __future__ import unicode_literals
+import datetime
+import hashlib
+import random
+import string
 
+from django.utils import timezone
 from django.db import migrations
 
 # Hack
@@ -92,7 +97,15 @@ def transfer_old_invitations_to_new_tables(apps, schema_editor):
         )
         if new_inv.invitation_type in ['ci', 'cp']:
             new_inv.invitation_type = 'C'
-        new_inv.refresh_keys(force_new_key=True)
+
+        # Generate keys, custom methods are not loaded here
+        salt = ''
+        for i in range(5):
+            salt += random.choice(string.ascii_letters)
+        salt = salt.encode('utf8')
+        invitationsalt = new_inv.last_name.encode('utf8')
+        new_inv.invitation_key = hashlib.sha1(salt + invitationsalt).hexdigest()
+        new_inv.key_expires = timezone.now() + datetime.timedelta(days=365)
         new_inv.save()
 
         if invitation.cited_in_submission: