Replace U2F with WebAuthn (#1392)

* Replace U2F with WebAuthn

* Imports

* Fix backwards compatibility

* Add explanatory comment

* Fix tests

src/pretix/base/migrations/0134_auto_20190909_1042.py

@@ -0,0 +1,35 @@
+# Generated by Django 2.2.4 on 2019-09-09 10:42
+
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+import pretix.base.models.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0133_auto_20190830_1513'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='WebAuthnDevice',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False)),
+                ('name', models.CharField(max_length=64)),
+                ('confirmed', models.BooleanField(default=True)),
+                ('credential_id', models.CharField(max_length=255, null=True)),
+                ('rp_id', models.CharField(max_length=255, null=True)),
+                ('icon_url', models.CharField(max_length=255, null=True)),
+                ('ukey', models.TextField(null=True)),
+                ('pub_key', models.TextField(null=True)),
+                ('sign_count', models.IntegerField(default=0)),
+                ('user', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, to=settings.AUTH_USER_MODEL)),
+            ],
+            options={
+                'abstract': False,
+            },
+        ),
+    ]