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/dashboards.py

@@ -2,7 +2,6 @@ from decimal import Decimal
 
 import pytz
 from django.contrib.contenttypes.models import ContentType
-from django.core.urlresolvers import reverse
 from django.db.models import (
     Count, Exists, IntegerField, Max, Min, OuterRef, Q, Subquery, Sum,
 )
@@ -10,6 +9,7 @@ from django.db.models.functions import Coalesce, Greatest
 from django.dispatch import receiver
 from django.shortcuts import render
 from django.template.loader import get_template
+from django.urls import reverse
 from django.utils import formats
 from django.utils.formats import date_format
 from django.utils.html import escape