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

doc/development/api/customview.rst

@@ -64,7 +64,7 @@ Similarly, there is ``organizer_permission_required`` and ``OrganizerPermissionR
 event-related views, there is also a signal that allows you to add the view to the event navigation like this::
 
 
-    from django.core.urlresolvers import resolve, reverse
+    from django.urls import resolve, reverse
     from django.dispatch import receiver
     from django.utils.translation import ugettext_lazy as _
     from pretix.control.signals import nav_event