[Cashtab] Enable min fee sends for users with 1 million grumpy

Summary:
Add a new key to settings, minFeeSends, that sets tx fees to 1.0 satoshis per byte. For now, gate the feature so it is available only to holders of 1 million GRP.

User must have correct token balance and have chosen this setting for these sends to be enabled.

Update validation and migration functions for cashtabSettings accordingly. Add integration tests.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D15575