Upgrade to Django 2.1 (#710)

* Upgrade to Django 2.0

* more models

* i18n foo

* Update setup.py

* Fix Sentry exception PRETIXEU-JC

* Enforce slug uniqueness

* Import sorting

* Upgrade to Django 2.1

* Travis config

* Try to fix PostgreSQL failure

* Smaller test matrix

* staticfiles→static

* Include request in all authenticate() calls

src/pretix/base/migrations/0098_auto_20180731_1243.py

@@ -0,0 +1,56 @@
+# Generated by Django 2.0.7 on 2018-07-31 12:43
+
+import django.core.validators
+import django.db.models.deletion
+from django.conf import settings
+from django.db import migrations, models
+
+import pretix.base.validators
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pretixbase', '0097_auto_20180722_0804'),
+    ]
+
+    operations = [
+        migrations.AlterModelOptions(
+            name='logentry',
+            options={'ordering': ('-datetime', '-id')},
+        ),
+        migrations.AlterField(
+            model_name='orderpayment',
+            name='fee',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.SET_NULL, related_name='payments', to='pretixbase.OrderFee'),
+        ),
+        migrations.AlterField(
+            model_name='organizer',
+            name='slug',
+            field=models.SlugField(help_text='Should be short, only contain lowercase letters, numbers, dots, and dashes. Every slug can only be used once. This is being used in URLs to refer to your organizer accounts and your events.', unique=True, validators=[django.core.validators.RegexValidator(message='The slug may only contain letters, numbers, dots and dashes.', regex='^[a-zA-Z0-9.-]+$'), pretix.base.validators.OrganizerSlugBlacklistValidator()], verbose_name='Short form'),
+        ),
+        migrations.AlterField(
+            model_name='staffsession',
+            name='user',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, to=settings.AUTH_USER_MODEL),
+        ),
+        migrations.AlterField(
+            model_name='staffsessionauditlog',
+            name='impersonating',
+            field=models.ForeignKey(blank=True, null=True, on_delete=django.db.models.deletion.PROTECT, to=settings.AUTH_USER_MODEL),
+        ),
+        migrations.AlterField(
+            model_name='staffsessionauditlog',
+            name='session',
+            field=models.ForeignKey(on_delete=django.db.models.deletion.PROTECT, related_name='logs', to='pretixbase.StaffSession'),
+        ),
+        migrations.AlterField(
+            model_name='user',
+            name='locale',
+            field=models.CharField(choices=[('en', 'English'), ('de', 'German'), ('de-informal', 'German (informal)'), ('nl', 'Dutch'), ('da', 'Danish'), ('tr', 'Turkish'), ('pt-br', 'Portuguese (Brazil)')], default='en', max_length=50, verbose_name='Language'),
+        ),
+        migrations.AlterUniqueTogether(
+            name='event',
+            unique_together={('organizer', 'slug')},
+        ),
+    ]