Add ability to hide answers in Ponder

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