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/waitinglist.py

@@ -43,10 +43,11 @@ class WaitingListEntry(LoggedModel):
         'Voucher',
         verbose_name=_("Assigned voucher"),
         null=True, blank=True,
-        related_name='waitinglistentries'
+        related_name='waitinglistentries',
+        on_delete=models.CASCADE
     )
     item = models.ForeignKey(
-        Item, related_name='waitinglistentries',
+        Item, related_name='waitinglistentries', on_delete=models.CASCADE,
         verbose_name=_("Product"),
         help_text=_(
             "The product the user waits for."
@@ -54,7 +55,7 @@ class WaitingListEntry(LoggedModel):
     )
     variation = models.ForeignKey(
         ItemVariation, related_name='waitinglistentries',
-        null=True, blank=True,
+        null=True, blank=True, on_delete=models.CASCADE,
         verbose_name=_("Product variation"),
         help_text=_(
             "The variation of the product selected above."