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/models/checkin.py

@@ -8,12 +8,12 @@ from pretix.base.models import LoggedModel
 
 
 class CheckinList(LoggedModel):
-    event = models.ForeignKey('Event', related_name='checkin_lists')
+    event = models.ForeignKey('Event', related_name='checkin_lists', on_delete=models.CASCADE)
     name = models.CharField(max_length=190)
     all_products = models.BooleanField(default=True, verbose_name=_("All products (including newly created ones)"))
     limit_products = models.ManyToManyField('Item', verbose_name=_("Limit to products"), blank=True)
     subevent = models.ForeignKey('SubEvent', null=True, blank=True,
-                                 verbose_name=pgettext_lazy('subevent', 'Date'))
+                                 verbose_name=pgettext_lazy('subevent', 'Date'), on_delete=models.CASCADE)
     include_pending = models.BooleanField(verbose_name=pgettext_lazy('checkin', 'Include pending orders'),
                                           default=False,
                                           help_text=_('With this option, people will be able to check in even if the '
@@ -157,7 +157,7 @@ class Checkin(models.Model):
     """
     A check-in object is created when a person enters the event.
     """
-    position = models.ForeignKey('pretixbase.OrderPosition', related_name='checkins')
+    position = models.ForeignKey('pretixbase.OrderPosition', related_name='checkins', on_delete=models.CASCADE)
     datetime = models.DateTimeField(default=now)
     nonce = models.CharField(max_length=190, null=True, blank=True)
     list = models.ForeignKey(