feature: add goerli support

[bitpshr]

This pull request adds goerli network support.

Resolves #6249

[metamaskbot]

Builds ready [b940757]: chrome, firefox, edge, opera

[whymarrh]

LGTM, one small nitpick: can we drop the changes to the package-lock.json file as there are no changes to the package.json file in this PR?

[bitpshr]

Thanks @whymarrh, good catch. Updated.

[metamaskbot]

Builds ready [b711734]: chrome, firefox, edge, opera

[whymarrh]

@tmashuang would you be able to give this a QA pass as well? It seems good to me.

[tmashuang]

Will need to add the network list to the the buyEth deposit modal

metamask-extension/ui/app/components/app/modals/deposit-ether-modal.js

Line 136 in ac39a9b

const isTestNetwork = ['3', '4', '42'].find(n => n === network)

and provide a url for the test network

metamask-extension/app/scripts/lib/buy-eth-url.js

Line 14 in 98a2fb4

function getBuyEthUrl ({ network, amount, address, service }) {

I recommend https://bridge.goerli.com/ from
https://github.com/goerli/testnet/blob/master/README.md#meta-data-görli but requires eth from other test networks, could use the other ones recommended.

Edit: Couldn't get the bridge to work.
https://goerli-faucet.slock.it/ provides 0.05/tx
https://faucet.goerli.mudit.blog/ will need a social media link with an address as the message

[tmashuang]

This PR, #6444 also touched the network-dropdown.js will need to resolve those conflicts.

Added commit

[metamaskbot]

Builds ready [180f1b8]: chrome, firefox, edge, opera

[whymarrh]

Seems legit (cc @bitpshr)