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/control/views/checkin.py

@@ -1,10 +1,10 @@
 import dateutil.parser
 from django.contrib import messages
-from django.core.urlresolvers import reverse
 from django.db import transaction
 from django.db.models import Max, OuterRef, Subquery
 from django.http import Http404, HttpResponseRedirect
 from django.shortcuts import get_object_or_404, redirect
+from django.urls import reverse
 from django.utils.functional import cached_property
 from django.utils.timezone import is_aware, make_aware, now
 from django.utils.translation import ugettext_lazy as _