Upgrade to Django 2.2 and modern DRF and py.test (#1246)

* Upgrade django and stuff

* Update to Django 2.2 and recent versions of similar packages

* Provide explicit orderings to all models used in paginated queries

* Resolve naive datetime warnings in test suite

* Deal with deprecation warnings

* Fix sqlparse version

src/pretix/plugins/badges/models.py

@@ -52,3 +52,6 @@ class BadgeItem(models.Model):
                                 on_delete=models.CASCADE)
     layout = models.ForeignKey('BadgeLayout', on_delete=models.CASCADE, related_name='item_assignments',
                                null=True, blank=True)
+
+    class Meta:
+        ordering = ('id',)