remote_billing: Make "plan management" always available.

Just shows a config error page if the bouncer is not enabled. Uses a new
endpoint for this so that it can work nicely for both browser and
desktop app clients.
It's necessary, because the desktop app expects to get a json response
with either an error or billing_access_url to redirect to. Showing a
nice config error page can't be done via the json error mechanism, so
instead we just serve a redirect to the new error page, which the app
will open in the browser in a new window or tab.

zproject/urls.py

@@ -99,6 +99,7 @@ from zerver.views.push_notifications import (
     remove_android_reg_id,
     remove_apns_device_token,
     self_hosting_auth_json_endpoint,
+    self_hosting_auth_not_configured,
     self_hosting_auth_redirect_endpoint,
     send_test_push_notification_api,
 )
@@ -828,6 +829,10 @@ urls += [
         self_hosting_auth_redirect_endpoint,
         name="self_hosting_auth_redirect_endpoint",
     ),
+    path(
+        "self-hosted-billing/not-configured/",
+        self_hosting_auth_not_configured,
+    ),
     rest_path(
         "json/self-hosted-billing",
         GET=self_hosting_auth_json_endpoint,