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.

(cherry picked from commit f6d526f798)

zerver/lib/test_helpers.py

@@ -528,6 +528,9 @@ def write_instrumentation_reports(full_suite: bool, include_webhooks: bool) -> N
             # TODO: This endpoint and the rest of its system are a work in progress,
             # we are not testing it yet.
             "self-hosted-billing/",
+            # This endpoint only returns 500 and 404 codes, so it doesn't get picked up
+            # by find_pattern above and therefore needs to be exempt.
+            "self-hosted-billing/not-configured/",
             *(webhook.url for webhook in WEBHOOK_INTEGRATIONS if not include_webhooks),
         }