User onboarding should introduce the user to using the product, as well as provide email verification service

New User Experience suggestion:
1. `keybase` or web client creates a key for the user, and prompts for email address (Ideally using template as in https://github.com/keybase/keybase-issues/issues/524)
2. keybase.io encrypts a message to the public key with a random sentence/phrase/words/string and sends via email to the email address provided by the user.
3. The user must decrypt the message with either web page or `keybase` command line client (introducing them to using the product as well as providing verification of key ownership/proper installation/email address receipt)
4. User returns to keybase.io and enters the sent random sentence/phrase/words/string to confirm their account

Start by reviewing the web client, keybase.io, and keybase command-line client portions involved in new-user onboarding and email verification. Trace how a new user is created, how email messages are sent, and how verification is confirmed; done means the proposed flow works across those entry points and introduces the user to the product.

Written by the indexing model from the issue text.