Include QR code alongside invoice request copy button

**Is your feature request related to a problem? Please describe.**
When a payment invoice is created, the merchant has to copy the text string and send this to the customer.

When the customer goes to pay, they have the option to either enter the payment request string, or they can scan a QR code to enter the text string quicker.

There isn't a built in QR code generated at the payment request page, so the QR code has to either be generated with an external app (or implemented custom by merchant in their web ui), or the customer has to enter the text.

If at a shop till, copying text from one device to another without a QR code is complex.

**Describe the solution you'd like**
Beside the 'copy string' button, add a QR code generation modal button, enabling the customer to more quickly scan the payment request.

**Describe alternatives you've considered**
Use 3rd party merchant apps.

**Additional context**
This is an UX improvement request.

Start at the payment request page and locate the existing copy-string button and invoice display. Add the requested QR-code modal beside it, then verify that a customer can scan the displayed payment request instead of copying or typing it.

Written by the indexing model from the issue text.