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/presale/views/locale.py

@@ -11,7 +11,7 @@ from .robots import NoSearchIndexViewMixin
 class LocaleSet(NoSearchIndexViewMixin, View):
 
     def get(self, request, *args, **kwargs):
-        url = request.GET.get('next', request.META.get('HTTP_REFERER', '/'))
+        url = request.GET.get('next', request.headers.get('Referer', '/'))
         url = url if is_safe_url(url, allowed_hosts=[request.get_host()]) else '/'
         resp = HttpResponseRedirect(url)