corporate: Add prototype authentication system for self-hosters.

This makes it possible for a self-hosted realm administrator to
directly access a logged-page on the push notifications bouncer
service, enabling billing, support contacts, and other administrator
for enterprise customers to be managed without manual setup.

zproject/urls.py

@@ -92,6 +92,7 @@ from zerver.views.push_notifications import (
     add_apns_device_token,
     remove_android_reg_id,
     remove_apns_device_token,
+    self_hosting_auth_redirect,
     send_test_push_notification_api,
 )
 from zerver.views.reactions import add_reaction, remove_reaction
@@ -820,6 +821,10 @@ urls += [
     path("policies/<slug:article>", policy_documentation_view),
 ]
 
+urls += [
+    path("self-hosted-billing/", self_hosting_auth_redirect, name="self_hosting_auth_redirect"),
+]
+
 if not settings.CORPORATE_ENABLED:  # nocoverage
     # This conditional behavior cannot be tested directly, since
     # urls.py is not readily reloaded in Django tests. See the block