Restored support to oldest Django versions

pwa/urls.py

@@ -1,10 +1,10 @@
-from django.urls import path
+from django.conf.urls import url
 
-from .views import Manifest, ServiceWorker, OfflineView
+from .views import manifest, service_worker, offline
 
 # Serve up serviceworker.js and manifest.json at the root
 urlpatterns = [
-    path('serviceworker.js', ServiceWorker.as_view(), name='serviceworker'),
-    path('manifest.json', Manifest.as_view(), name='manifest'),
-    path('offline', OfflineView.as_view(), name='offline')
+    url('^serviceworker.js$', service_worker, name='serviceworker'),
+    url('^manifest.json$', manifest, name='manifest'),
+    url('^offline/$', offline, name='offline')
 ]