[web] Create a new 'secondary device' screen that will generate the QR code

Summary:
This is the page that will display the QR code, alongside the instructions on where to find the scanner on the native app. These are the designs shown on Figma.

For now, the data encoded in the QR code is just a link, but I will work on supporting 'deep-linking' from scanning with an external camera in ENG-4648. So ultimately that will be changed.

Addresses ENG-4478

Depends on D8818

Test Plan:
Please see the video below

Reviewers: atul, ginsu, tomek, varun, ted

Reviewed By: tomek

Subscribers: ashoat, ted

Differential Revision: https://phab.comm.dev/D8821