Restore Ponder question commenting

Summary: Ref T3373. Same issues as the other commenting patch; it's huge and the JS is a bit buggy. Backend is fine, though.

Test Plan: Made some comments on a question.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3373

Differential Revision: https://secure.phabricator.com/D6610