[Cashtab] Remove separate block explorers for token txs

Summary: In Ticker.js, removed currency.tokenExplorerUrl, and replaced value of currency.blockExplorerUrl to explorer.be.cash. In useBCH.js and useBCH.test.js, replaced each instance of currency.tokenExplorerUrl with currency.blockExplorerUrl. Related to task T2215.

Test Plan:
cd web/cashtab && npm start

From the terminal:
Run grep -r 'tokenExplorerUrl' src/ - observe that nothing is returned.

From the wallet tab:
Click on an XEC transaction, ensure that the link leads to explorer.be.cash.
Click on an eToken transaction, ensure that the linke leads to explorer.be.cash.
Send eToken, click on transaction as above
Receive eToken, click on transaction as above

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11428