[Cashtab] Update block explorer to explorer.e.cash

Summary: Related to T2602. Replaced explorer.be.cash with explorer.e.cash.

Test Plan:
cd web/cashtab && npm start
ensure there are both XEC and eToken txs in recent TxHistory
expand the tx collapse for an XEC tx
click 'View on e.cash'
ensure that the link brings user to correct tx in explorer

expand the tx collapse for an eToken tx
click on 'View on e.cash'
ensure that the link brings the user to correct tx in explorer

grep -r 'explorer.be.cash' src/ should return no results

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11853