Fix PonderQuestionCreateMailReceiver accessing non-existing PonderQuestionTransaction constants

Summary:
The transaction types TYPE_TITLE and TYPE_CONTENT were removed from PonderQuestionTransaction in rPdff028c4907dd1959859733ea0d947f244559e7f.
Thus update PonderQuestionCreateMailReceiver to use the corresponding modular transactions PonderQuestionTitleTransaction and PonderQuestionContentTransaction instead.

Test Plan: Carefully read the code, grep the code base, use the test case in D25656#18162.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25656