[ecash-herald] Show less info in daily msgs

Summary:
Need to keep track of "impact" in updates here. While the detail is useful impact for devs and power users -- e.g., changes in Agora volume are a good KPI to watch for the impact of this feature -- we need to cut out as much info as possible for the updates to be interesting to users. This will also support plans to prep these daily summaries for twitter.

Reduce the number of tokens we show. Do not show invalid token txs.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D17176