Implement user blocking

Keybase should allow user blocking, which would presently lock out the `track` function and prevent the blockee from tracking the blocker.

Blocking is especially important since @keybase.io email addresses and web-based messaging are upcoming features and can easily facilitate harassment. At that point, blocking should lock out messages via the website and email redirection from the email addresses attached to the blockee's profile & public key.

The issue names no files, tests, or entry points; begin by locating the existing track, messaging, email-redirection, and profile/public-key flows. Define the blocking model and its behavior across those flows, then add coverage showing that a blocked user cannot track or contact the blocker.

Written by the indexing model from the issue text.