[Cashtab] Replace zxing with html5-qrscode

Summary: Follow implementation of scanapp.org, support zoom, improve modal style. Seems to improve scanning behavior.

Test Plan:
This is deployed at cashtab.io. Try scanning QR codes. npm test

This QR code in particular:

• cashtab.com struggles to scan it
• cashtab.io (should) scan it instantly

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18486