[Cashtab] Change dust param from BCHA to sats

Summary:
The currency.dust param has been set to '0.00000546' to reflect the minimum amount of BCHA that the app will send.

To support potential decimal place moves, this figure should be in sats.

This diff deletes the currency.dust parameter, replaces it with currency.dustSats, and parses this value for the arbitrary currency.cashDecimals figure wherever it is used in the app. Unit tests are included to ensure consistent application.

Test Plan:
npm test
npm start
Try to send 0.00000545 BCHA and note error msg is correct

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9495