[Cashtab] Use capacitor barcode scanner to dramatically simplify and improve scanning of qr codes

Summary: Cashtab has had a series of custom scanner implementations. Capacitor now has a well maintained lib that does just what we want. Use this to standardize and improve the scanning experience across web and mobile.

Test Plan: npm test, check out cashtab.io, run app with ./start-android and scan something

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18759