Added unique constraint on email

ECOM-7085

(cherry picked from commit d141eb15)

common/djangoapps/student/migrations/0010_auto_20170207_0458.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('student', '0009_auto_20170111_0422'),
+    ]
+
+    operations = [
+        migrations.RunSQL(
+            "create unique index email on auth_user (email);",
+            "drop index email on auth_user;"
+        )
+    ]

openedx/core/lib/django_courseware_routers.py

@@ -45,13 +45,15 @@ class StudentModuleHistoryExtendedRouter(object):
             return False
         return None
 
-    def allow_migrate(self, db, model):  # pylint: disable=unused-argument
+    def allow_migrate(self, db, app_label, model_name=None, **hints):  # pylint: disable=unused-argument
         """
         Only sync StudentModuleHistoryExtended to StudentModuleHistoryExtendedRouter.DATABASE_Name
         """
-        if self._is_csmh(model):
-            return db == self.DATABASE_NAME
-        elif db == self.DATABASE_NAME:
+        if model_name is not None:
+            model = hints.get('model')
+            if self._is_csmh(model):
+                return db == self.DATABASE_NAME
+        if db == self.DATABASE_NAME:
             return False
 
         return None