[Cashtab] Move explorer config out of Ticker

Summary: Moving the explorer related params out of Ticker.js and into its own config file.

Test Plan:
grep -r currency.blockExplorerUrl src/
grep -r currency.blockExplorerUrlTestnet src/
grep -r currency.pdfReceiptUrl src/
npm test
npm start
click on any transaction in the Tx History tab and then click the View on e.cash link and ensure the appropriate tx is displayed in explorer.e.cash
click on any transaction in the Tx History tab and then click the Receipt links and ensure a Blockchair powered transaction receipt PDF is displayed

Reviewers: bytesofman, #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14121