[Cashtab] Use dust for dust throughout app

Summary:
T3539

Cashtab has used 550 sats for the "min send" XEC value and actual dust (546 sats) for token outputs. The original rationale for this: bch-api was not great at separating token utxos from non-token utxos, and this provided another fall-back insurance on separating utxos.

Was a bad "solution" at the time and really should be out by now.

Test Plan: npm start

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15984