Make (user, client) unique on UserConsent

We assume this combination is unique with our get and get_or_create
calls.

oidc_provider/migrations/0006_unique_user_client.py

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('oidc_provider', '0005_token_refresh_token'),
+    ]
+
+    operations = [
+        migrations.AlterUniqueTogether(
+            name='userconsent',
+            unique_together=set([('user', 'client')]),
+        ),
+    ]

oidc_provider/models.py

@@ -89,5 +89,5 @@ class Token(BaseCodeTokenModel):
 
 
 class UserConsent(BaseCodeTokenModel):
-
-    pass
+    class Meta:
+        unique_together = ('user', 'client')