Use PHUIRemarkupPreviewPanel in Ponder

Summary:
Ref T3578. Ref T3671. Depends on D6673. Use `PHUIRemarkupPreviewPanel` (introduced in D6673) to provide question create/edit and answer edit previews in Ponder.

Then delete a million lines of duplicate code.

Test Plan: Edited a question; edited an answer. Saw live previews.

Reviewers: btrahan, Firehed

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3578, T3671

Differential Revision: https://secure.phabricator.com/D6674