Implement block lists. They work like friend lists but are unidirectional,
but take effect in both directions (blocker and blockee can not see one
another on the site -- except admin users can always see all users).
* Profile page says 404
* User gallery says 404
* User search page filters out blocked users
* Compose endpoint blocks sending messages to blocked users (except admin)
* Site Gallery filters photos by blocked (and uncertified) users
* Inbox page hides chat list for blocked users (can still read the chat
history if you have a link to the old thread)
