[electrum] update block explorers

Summary:
[t]explorer.bitcoinabc.org is going to be retired soon.

explorer.viawallet.com has been a redirect to expplorer.coinex.com for a
while now.

Add texplorer.e.cash

Users who previously selected [t]explorer.bitcoinabc.org will switch
to using the default explorer, which is [t]explorer.e.cash

For users who selected explorer.viawallet.com this diff adds some code to
automatically switch to coinex.explorer.com.

Test Plan:
Test that all the explorer still work, both for transactions (History
tab) and for address URLs (Address tab).

Edit ~/.electrum-abc/config, set "block_explorer": "ViaWallet",
bump the version in version.py to trigger the update,
check that the "CoinEx" explorer is selected in the configuration menu.
Check that the line in config was
updated to "block_explorer": "CoinEx"

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D16875