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