Add cookie-attribution based authentication

[pibara-utopian]

Outline described here:

https://steemit.com/utopian-io/@mattockfs/suggestion-add-cookie-attribution-based-authentication-to-steem-keychain

Basic idea: standardize on a div format and a custom_json for cookie-attribution based authentication, and implement in Steem Keychain.

[stoodkev]

This looks to me like something that can already be done on the client side using existing Keychain features.