[web] Add QR auth handler logic

Summary:
Added QR code auth logic on web. Basically web counterpart of parent diffs in this stack.

Depends on D11330

Test Plan: Repeated test plan from parent diffs in this stack (mainly D11301 and D11330), but used web as a secondary device.

Reviewers: kamil, michal

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11331