[Cashtab] replace legacy DUST param with constant

Summary:
Cashtab has an artefact parameter DUST leftover from mint.bitcoin.com days. This diff removes the parameter and replaces it with the constant currency.dust. A unit test is added to ensure this new variable is parsed the same way as the previous one.

This improvement is a step toward refactoring all functions that deal with the smallest base unit to use the BigNumber.js library. This will prepare the app to accept arbitrary base units.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9102