verifyAuthRequest name lookup URL is on mainnet

[hstove]

Although mainnet is live, BNS endpoints are not deployed to mainnet yet. So, if an app uses the latest @stacks/auth version, and a user logs in with a username, then verifyAuthRequest will fail (this is called by default in all auth flows).

Options to fix temporarily:

• Apps can specify appConfig.coreNode to https://core.blockstack.org - this is a decent fix for as long as we don't have BNS endpoints or a new subdomain registrar
• Update @stacks/auth default URL to use https://core.blockstack.org - and we can revert that once BNS is live

We should be careful to test this - the API is not currently 1-for-1, for example, /v1/names/:name returns a stacks address (it used to be a BTC address), so there might be slight changes needed to support this change. Part of that might need to be on the authenticator, and some might be in @stacks/auth itself.