[token-server] Add XEC faucet to Cashtab

Summary:
Add XEC airdrops for new Cashtab wallets.

Only newly-created wallets are eligible for airdrops.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Klakurka, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16615