Implement OAuth2 provider (#927)

- [x] Application management
  - [x] Link
  - [ ] Tests
- [x] Authorize flow
  - [x] Tests
- [x] Refresh token handling
  - [x] Tests
- [x] Revocation endpoint
  - [x] Tests
  - [x] Mitigate: https://github.com/jazzband/django-oauth-toolkit/issues/585
- [x] API authenticator / permission driver
  - [x] Test
- [x] Enforce organizer restriction
  - [x] Tests
- [x] Enforce scope restriction
  - [x] Tests
- [x] Show current applications to user
  - [x] Revoke
  - [x] Tests
- [x] Log new authorizations
  - [x] notify user
- [x] Ensure other grant types are not available
- [x] Documentation
- [x] check if revoking access toking, then refreshing gets rid of organizer constraint
- [x] Show logentry foo

src/pretix/base/migrations/0093_auto_20180528_1432.py

@@ -21,7 +21,6 @@ def set_pids(apps, schema_editor):
 
 
 class Migration(migrations.Migration):
-
     dependencies = [
         ('pretixbase', '0092_auto_20180511_1224'),
     ]

src/pretix/base/migrations/0094_auto_20180604_1119.py

@@ -0,0 +1,15 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.13 on 2018-06-04 11:19
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0093_auto_20180528_1432'),
+        ('pretixapi', '0001_initial')
+    ]
+
+    operations = [
+    ]

src/pretix/base/migrations/0095_auto_20180604_1129.py

@@ -0,0 +1,21 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.13 on 2018-06-04 11:29
+from __future__ import unicode_literals
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0094_auto_20180604_1119'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='logentry',
+            name='oauth_application',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT,
+                                    to='pretixapi.OAuthApplication'),
+        ),
+    ]