[Cashtab] Patch send to many with improved validation

Summary:
Broke send to many in D15776, as the send button will be disabled even if user input is correct.

Update validation function for disabling send button. Add an integration test for multisend to prevent this kind of oversight in the future.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15816