Re-configure timezones for API and Admin

Use UTC as Django default timezone so that Django Rest Framework will
use it for the API endpoints, but override the timezone in Django Admin
to still be Europe/Helsinki with a middleware.

parkkihubi/middleware.py

@@ -0,0 +1,31 @@
+import contextlib
+from functools import lru_cache
+
+from django.conf import settings
+from django.urls import reverse
+from django.utils import timezone
+
+# No-op context manager
+#
+# Usable like the one coming in Python 3.7, see
+# https://github.com/python/cpython/commit/0784a2e5b174d2dbf7b144d480559e650c5cf64c
+nullcontext = contextlib.ExitStack
+
+
+class AdminTimezoneMiddleware:
+    def __init__(self, get_response):
+        self.get_response = get_response
+
+    def __call__(self, request):
+        tz_overrider = nullcontext()
+        if request.path.startswith(get_admin_url_path_prefix()):
+            admin_tz = getattr(settings, 'ADMIN_TIME_ZONE', None)
+            if admin_tz:
+                tz_overrider = timezone.override(admin_tz)
+        with tz_overrider:
+            return self.get_response(request)
+
+
+@lru_cache()
+def get_admin_url_path_prefix():
+    return reverse('admin:index')

parkkihubi/settings.py

@@ -87,10 +87,11 @@
 ##############
 # Middleware #
 ##############
-MIDDLEWARE_CLASSES = [
+MIDDLEWARE = [
     'django.contrib.sessions.middleware.SessionMiddleware',
     'django.middleware.common.CommonMiddleware',
     'django.middleware.csrf.CsrfViewMiddleware',
+    'parkkihubi.middleware.AdminTimezoneMiddleware',
     'django.contrib.auth.middleware.AuthenticationMiddleware',
     'django.contrib.auth.middleware.SessionAuthenticationMiddleware',
     'django.contrib.messages.middleware.MessageMiddleware',
@@ -133,7 +134,8 @@
 # Languages & Localization #
 ############################
 LANGUAGE_CODE = 'en'
-TIME_ZONE = 'Europe/Helsinki'
+TIME_ZONE = 'UTC'
+ADMIN_TIME_ZONE = 'Europe/Helsinki'
 USE_I18N = True
 USE_L10N = True
 USE_TZ = True