Create passthrough-keyring for Brave

[danfinlay]

When running in Brave, should auto-detect a Geth instance running on localhost:8545 with unlocked accounts.

• Should show any unlocked accounts as accounts in this passthrough keyring.
• Should allow signing with these accounts.

Stretch goals:

• Initialize account nicknames using RPC: db_getString, params: ['braveEthWallet', 'wallets']. Will return data of form: [{name: "Account 1", address: "0xabcdef…"}].