[Cashtab] Format XEC on send form

Summary: To avoid confusion, as the user inputs an amount into the send form, they are shown the value formatted to their locale in the space beneath the input area.

Test Plan:
cd web/cashtab && npm start
navigate to send tab
input an amount with more tha 3 digits
ensure formatting is taking place
in browser settings, change preferred language
navigate to send tab
input an amount that is more than 3 digits
ensure formatting meets the requirements for your selected locale

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10776