Summary: Ref T9173, adds basic hide support for answers. Answer authors and Moderators can hide answers, unhide them.
Test Plan: Hide answer, log into other account, see hidden message. Mark as visible, see answer again.
Reviewers: epriestley
Reviewed By: epriestley
Subscribers: Korvin
Maniphest Tasks: T9173
Differential Revision: https://secure.phabricator.com/D13894
