Unlock button asks user to sign message to log into account

[lsaether]

As a decentralized project we should use a decentralized log-in. Metamask is not perfect, but it's the best we've got. We can propose to Metamask that the user signs a message. We can use the signature of the message to "unlock" their Arena account.

Goal
The user starts on the landing page. They already have a Convergent-Id. To log into Arena, they click the "UNLOCK" button on the right hand side of the AppBar. This will prompt Metamask for them to sign a message. We use the signed message as verification and log them into the Convergent-Id account associated with that address. This will reroute them to their initial Dashboard page with sidebar.

Deliverables

1. The Unlock button prompts for signature.
2. the Unlock button turns into a blank image. (To fill in is separate issue Unlock button turns into Convergent-Id account or a blocky identicon after login #19 )

References:
https://www.toptal.com/ethereum/one-click-login-flows-a-metamask-tutorial
https://chibifighters.io for a live example of sign in button using metamask