[ecash-herald] Give Cashtab XEC rewards their own line

Summary: These txs are currently listed like any other XEC tx. Pull them out of that list, where the info is not newsworthy, and instead include interesting info about these txs.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack, Fabien

Reviewed By: #bitcoin_abc, emack, Fabien

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16849