[Cashtab] Move chronik pagination config out of Ticker

Summary:
Moving txHistoryPageSize and txHistoryCount out of Ticker.js and into the existing config/chronik.js.

For reference:

• txHistoryPageSize is a param for Cashtab to avoid exceeding chronik's maximum of 25 txs per chronik.script().history() call
• txHistoryCount is the number of most recent txs from chronik to be rendered in Cashtab's Tx History tab

Test Plan:
grep -r currency.txHistoryPageSize src/ and ensure no outputs
grep -r currency.txHistoryCount src/ and ensure no outputs
npm test
npm start and ensure no errors with Tx History tab
send and receive some XECs and ensure the latest txs are displayed at the top of the Tx History tab

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14120