Stop using Django's SlugField to avoid conflicting validation

src/pretix/base/migrations/0152_auto_20200510_1042.py

@@ -0,0 +1,35 @@
+# Generated by Django 3.0.6 on 2020-05-10 10:42
+
+from django.db import migrations, models
+import django_countries.fields
+import pretix.helpers.countries
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0151_auto_20200421_0737'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='event',
+            name='slug',
+            field=models.CharField(db_index=True, max_length=50),
+        ),
+        migrations.AlterField(
+            model_name='invoiceaddress',
+            name='country',
+            field=django_countries.fields.CountryField(countries=pretix.helpers.countries.CachedCountries, max_length=2),
+        ),
+        migrations.AlterField(
+            model_name='organizer',
+            name='slug',
+            field=models.CharField(db_index=True, max_length=50, unique=True),
+        ),
+        migrations.AlterField(
+            model_name='seat',
+            name='seat_guid',
+            field=models.CharField(db_index=True, max_length=190),
+        ),
+    ]