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/tests/base/test_models.py

@@ -1144,7 +1144,7 @@ class EventTest(TestCase):
         cl1.limit_products.add(i1)
 
         event2 = Event.objects.create(
-            organizer=self.organizer, name='Download', slug='ab1234',
+            organizer=self.organizer, name='Download', slug='ab54321',
             date_from=datetime.datetime(2013, 12, 26, tzinfo=datetime.timezone.utc)
         )
         event2.copy_data_from(event1)

src/tests/base/test_permissions.py

@@ -147,7 +147,7 @@ def test_event_permissions_multiple_teams(event, user):
     team2 = Team.objects.create(organizer=event.organizer, can_change_vouchers=True)
     team3 = Team.objects.create(organizer=event.organizer, can_change_event_settings=True)
     event2 = Event.objects.create(
-        organizer=event.organizer, name='Dummy', slug='dummy',
+        organizer=event.organizer, name='Dummy', slug='dummy2',
         date_from=now()
     )
     team1.members.add(user)