add correct options to explorer dropdown in testnet

[bordalix]

closes #326

@tiero what do you mean with "weird behaviour"?

[tiero]

This fixes the list of options for regtest, but the still we have following problem:

Run this extension in browser clean

1. Restore from menmonic
2. Settings > Networks pick Testnet
3. Setting > Explorers I asse Testnet as option still

[tiero]

Also, selectin Mempool is not using the right API URL, should be https://liquid.network/liquidtestnet/api instead of mainnet https://liquid.network/api

EDIT: also selecting back Blockstream will use the mainnet ones.

[bordalix]

@tiero 0c211bb should have fixed the issue of the explorers.

It's not a huge refactor, but you should review it.

Previously the app allowed for one to be on Testnet and use Blockstream mainnet explorer, which I believe was a bug.

There was a specific option called "Testnet" that used Blockstream testnet explorer, and there wasn't support for Mempool testnet explorer.

In a nutshell, now we have:

mainnet

• blockstream
  • electrsURL: 'https://blockstream.info/liquid',
  • esploraURL: 'https://blockstream.info/liquid/api',
• mempool
  • electrsURL: 'https://liquid.network',
  • esploraURL: 'https://liquid.network/api',
• custom

testnet

• blockstream
  • electrsURL: 'https://blockstream.info/liquidtestnet',
  • esploraURL: 'https://blockstream.info/liquidtestnet/api',
• mempool
  • electrsURL: 'https://liquid.network/liquidtestnet',
  • esploraURL: 'https://liquid.network/liquidtestnet/api',
• custom

regtest

• nigiri
  • electrsURL: 'http://localhost:5001',
  • esploraURL: 'http://localhost:3001',
• custom