[Cashtab] More improvements to QR code scanning

Summary:
Additional enhancements to QR code scanning

• autofocus
• larger modal on modal, use available area
• optional flashlight

Test Plan: npm test, this is deployed to cashtab.io, test scanner

Reviewers: #bitcoin_abc, Klakurka

Reviewed By: #bitcoin_abc, Klakurka

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D18494