[Cashtab] Better validation functions for token send or burn amounts

Summary:
T3496

Use a standardized and tested validation function for token send or burn amounts.

Currently, Cashtab uses a custom send validation function, and a bad burn validation function (does not check decimals). Use a better function for both since they have the same rules.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15826