Automatic redirect on invoice page after invoice is settled

[yzernik]

When a user makes a payment on an invoice, they have to manually click another button to access the file after the payment completes.

It would be better if the browser automatically redirects to the file after the payment settles.

This could be done with the following:

• Server Sent Events (SSE) from the server to the web browser client on the invoice page.
• Use the SubscribeInvoices RPC method with streaming response to listen for settled invoices in LND.
• When a settled invoice is found that matches the invoice payment hash, send a SSE event to the web frontend. This event can be handled by some Javascript code that tells the browser to redirect to the unlocked file page.