Include suggestions for explorer

This one is a lower priority issue that will expand a bit further.

Check this JSON feed:

https://chainid.network/chains.json

It's a bit list of current chains together with RPCs and Explorer links.

When we are editing a network, load this file and when complete add a suggestion drop-down to the RPC (filter out WSS) and block explorer.

Note that to get the block explorer link, just add /tx to the given link eg:

```{"name":"Ethereum Mainnet","chain":"ETH","network":"mainnet","icon":"ethereum","rpc":["https://mainnet.infura.io/v3/${INFURA_API_KEY}","wss://mainnet.infura.io/ws/v3/${INFURA_API_KEY}","https://api.mycryptoapi.com/eth","https://cloudflare-eth.com"],"faucets":[],"nativeCurrency":{"name":"Ether","symbol":"ETH","decimals":18},"infoURL":"https://ethereum.org","shortName":"eth","chainId":1,"networkId":1,"slip44":60,"ens":{"registry":"0x00000000000C2E074eC69A0dFb2997BA6C7d2e1e"},"explorers":[{"name":"etherscan","url":"https://etherscan.io","standard":"EIP3091"}]}```

This is the important section:

```"explorers":[{"name":"etherscan","url":"https://etherscan.io","standard":"EIP3091"}]}```

You'll need to change that to https://etherscan.io/tx

Can you add the dropdown as a feature to the InputView widget, as that will probably get re-used a lot.

Note that some testnets define faucets. Eventually we can work out a way to connect these up too. They would need to call the DappBrowserFragment to point it at the faucet, and add a 'copy wallet address' into the side menu, as a lot of faucets aren't dapps.

Note: this is lower priority than anything that Tomek or Sunil assign.