[Invoice] Simplify status update system.

[alloy]

• Add a (polling) subscript (live query) for an invoice’s status to InvoicePreview.
• Remove the use of NSNotification and just rely on the above
• This also means we can stop hooking into the webview delegate, to know an invoice was paid for, and start using the Safari VC and benefit of autofill (contact and credit card info).