Add management command to purge obsolete permissions from the database

docs/releases/2.0.rst

@@ -85,6 +85,7 @@ Common steps
 
 Migrate existing database schema with::
 
+    $ mayan-edms.py purge_permissions
     $ mayan-edms.py migrate
 
 During the migration several messages of stale content types can occur:

mayan/apps/permissions/management/commands/purge_permissions.py

@@ -0,0 +1,17 @@
+from __future__ import unicode_literals
+
+from django.core.management.base import BaseCommand
+
+from ...classes import Permission
+from ...models import StoredPermission
+
+
+class Command(BaseCommand):
+    help = 'Remove obsolete permissions from the database'
+
+    def handle(self, *args, **options):
+        for permission in StoredPermission.objects.all():
+            try:
+                Permission.get({'pk': '%s.%s' % (permission.namespace, permission.name)}, proxy_only=True)
+            except KeyError:
+                permission.delete()