secure avatar icons: identicons / jazzicons for zulip

[dckc]

to give members confidence about who they're chatting with, set zulip avatars to identicon / blockies style images based on on-chain identifier (registry UI or public key) and disable changing them.

• big visual change for even small change in identifier
• strong cryptographic relationship to identifier

jazzicon - npm
Wallet UI trick- mock the MetaMask account icon by using npm react-jazzicon : ethdev
renasboy/eth-address-icon: Generates a cool unique ethereum account address icon
ethereum/blockies: <1k library that generates blocky identicons

[dckc]

@leithaus suggested allowing people to mix their secure icon with artwork of their own.