[Cashtab] Move function only used on Send screen to helpers dir for Send screen

Summary:
Almost at the end of long, long effort to get rid of "wtf is this file" cashMethods.js

The last remaining function will go out after ts conversion diffs land

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17411