Discourse SSO (#560)

* Alter Discourse support to act as SSO provider.
Correct service group sync retry queueing.

* Correct default database enviroment variable names.

* Redirect to requested page after succesful login.

* Correct default redirect handling.
Correct attribute used to logout users on Discourse.
Improve logging messages to use parsed path on Discourse.

* Correct task retry syntax using bind=True.
Inherit from base exception so can catch TeamspeakErrors.

authentication/migrations/0009_auto_20161021_0228.py

@@ -0,0 +1,24 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.10.2 on 2016-10-21 02:28
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('authentication', '0008_set_state'),
+    ]
+
+    operations = [
+        migrations.RemoveField(
+            model_name='authservicesinfo',
+            name='discourse_username',
+        ),
+        migrations.AddField(
+            model_name='authservicesinfo',
+            name='discourse_enabled',
+            field=models.BooleanField(default=False),
+        ),
+    ]