R1:8f0c83606310

Add question and answer transactions to Ponder

Summary: Ref T3373.

Test Plan:
Ran this script, saw valid inserts:

<?php

$unguarded = AphrontWriteGuard::beginScopedUnguardedWrites();

id(new PonderQuestionTransaction())
->setAuthorPHID('herp')
->setObjectPHID('derp')
->setViewPolicy('blarp')
->setEditPolicy('goop')
->setTransactionType('beep')
->setContentSource(PhabricatorContentSource::newForSource('derp', array()))
->save();

id(new PonderAnswerTransaction())…