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>

model/websocket_message.go

@@ -81,6 +81,7 @@ const (
 	WebsocketEventDraftDeleted                        = "draft_deleted"
 	WebsocketEventAcknowledgementAdded                = "post_acknowledgement_added"
 	WebsocketEventAcknowledgementRemoved              = "post_acknowledgement_removed"
+	WebsocketEventHostedCustomerSignupProgressUpdated = "hosted_customer_signup_progress_updated"
 )
 
 type WebSocketMessage interface {