[ecash-herald] Better presentation of Cashtab rewards

Summary: Combine token and cachet reward summaries into the same section. Include a Cashtab header that links to Cashtab.

Test Plan:
npm test, review screenshots

Before:

After:

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D16856