[Cashtab] Show total send amount in modal for one-to-many send txs

Summary:
T2801

Show total amount of XEC user is about to send in a one-to-many tx in modal

Test Plan:
npm test

Send a one-to-many tx with "Send Confirmations" enabled in settings. Verify total and formatting.

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D14480