Add ability to reply to Ponder Answer emails

Summary: Ref T9068, Ref T3846. Maybe fixes both, but I'm having issues testing email replies in a sandbox. Moves answer feed/mail generation to the AnswerEditor, hides it in QuestionEditor.

Test Plan: Write an answer, see feed story, check /mail/ for mail generation.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T3846, T9068

Differential Revision: https://secure.phabricator.com/D13905