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/organizer.py

@@ -2,11 +2,11 @@ from django import forms
 from django.contrib import messages
 from django.core.exceptions import PermissionDenied
 from django.core.files import File
-from django.core.urlresolvers import reverse
 from django.db import transaction
 from django.db.models import Count
 from django.forms import inlineformset_factory
 from django.shortcuts import get_object_or_404, redirect
+from django.urls import reverse
 from django.utils.functional import cached_property
 from django.utils.translation import ugettext_lazy as _
 from django.views.generic import (