[ecash-herald] Add a countdown for the eCash halving

Summary: Add a simple countdown for the eCash halving

Test Plan:
npm test

Setting the halving height to 820,000 for testing:

• 1 block before the halving:

• Halving block:

• After halving block:

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15869