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/plugins/sendmail/signals.py

@@ -1,5 +1,5 @@
-from django.core.urlresolvers import resolve, reverse
 from django.dispatch import receiver
+from django.urls import resolve, reverse
 from django.utils.translation import ugettext_lazy as _
 
 from pretix.base.signals import logentry_display