Self-hosted in-product purchase (#21804)

* Self-hosted admins can purchase licenses in-app when `ServiceSettings,SelfHostedPurchase` is true (the default)
* Content Security Policy enables loading assets from `js.stripe.com/v3` when `ServiceSettings.SelfHostedPurchase` is true (the default).
* Add `hosted_customer` API subpath
* Add status of SelfHostedPurchase to telemetry config report.
* Support showing admins self-hosted invoices when `ServiceSettings.SelfHostedPurchase` is true (the default)

Co-authored-by: Conor Macpherson <116016004+ConorMacpherson@users.noreply.github.com>

i18n/en.json

@@ -2543,6 +2543,10 @@
     "id": "api.scheme.patch_scheme.license.error",
     "translation": "Your license does not support update permissions schemes"
   },
+  {
+    "id": "api.server.hosted_signup_unavailable.error",
+    "translation": "Portal unavailable for self-hosted signup."
+  },
   {
     "id": "api.server.license_up_for_renewal.error_generating_link",
     "translation": "Failed to generate the license renewal link"