[Cashtab] Upgrade BigNumber.js

Summary:
Still using "BN" from "slp-mdm" because never got around to getting rid of it. Kept thinking I could replace it totally with string methods. This is not practical for multiplication and division though.

For token quantities, we need BigNumber. And for agora quantities (nanosatoshis) we really need BigNumber.

So, we should use the latest version. And throw out the now-totally-unused slp-mdm.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17386