* Add a way for users to temporarily deactivate their accounts, in a
recoverable way should they decide to return later.
* A deactivated account may log in but have limited options: to
reactivate their account, permanently delete it, or log out.
* Fix several bugs around the display of comments, messages and
forum threads for disabled, banned, or blocked users:
* Messages (inbox and sentbox) will be hidden and the unread indicator
will not count unread messages the user can't access.
* Comments on photos and forum posts are hidden, and top-level threads
on the "Newest" tab will show "[unavailable]" for their text and
username.
* Your historical notifications will hide users who are blocked, banned
or disabled.
* Add a "Friends" tab to user profile pages, to see other users' friends.
* The page is Certification Required so non-cert users can't easily
discover any members on the site.
