Ledger account search & employ

[mikeshultz]

This adds a customized LedgerConnector for web3-react that will attempt to lookup accounts in an intelligent way. Goes like this:

1. Check Ledger Live derivation path, if first account has a balance, RETURN if not GOTO 2
2. Check Ledger chrome app derivation path, if first account has balance, RETURN, if not GOTO 3
3. Fallback to Ledger Live derivation path with an unfunded account

At some point it might be cool to add an account selector but I don't think any of these connectors support that right now?

This should replace #201
related issue: #21

[sparrowDom]

Nice, thanks for handling these different cases elegantly. Merging this in, and adding error states to the Login Widget.