R1:8c8211787894

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…