[Cashtab] Hide QR popup setting

Summary: Related to T2484 & T2506. Some users do not like the QR scanner popup, so an option has been added to the settings tab to turn off the scanner from appearing unless the button is clicked.

Test Plan:
cd web/cashtab && npm start
using devtools, set the screen width to be smaller than 769px
navigate to the send tab
observe that the QR scanner popup appears
navigate to a send token screen
observe that the QR scanner popup appears

navigate to the settings tab
check 'hide qr scanner'
navigate to the send tab
observe that the QR scanner does not appear
navigate to a send token screen
observe that the scanner popup does not appear

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11602