Refs #775 -- Pluggable authentication backends (#1447)

* Drag-and-drop: Force csrf_token to be present

* Rough design

* Missing file

* b.visble

* Forms

* Docs

* Tests

* Fix variable

src/pretix/base/migrations/0137_auto_20191015_1141.py

@@ -0,0 +1,17 @@
+# Generated by Django 2.2.4 on 2019-10-15 11:41
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ('pretixbase', '0136_auto_20190918_1742'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='user',
+            name='auth_backend',
+            field=models.CharField(default='native', max_length=255),
+        ),
+    ]