[Cashtab] Ignore immature coinbase utxos in sendXec function

Summary:
For Cashtab users with staking reward utxos in their wallet -- ignore unspendable staking reward utxos in

1. The sendXec function
2. The onMax function used to calculate the max sendable amount in a wallet

Test Plan:
npm test

In a wallet with staking rewards, npm start, verify result of onMax on SendXec screen does not include recent staking rwds

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15181