Take "emailCard" into account as "Card Created Date" in Board Export#103

Currently, when selecting “Card Created Date” as one of the fields to export in Board Export, it uses “createCard” or “copyCard” to extract that information.

However, for cards created via mail-to-board, those use “emailCard” which is not included in “Card Created Date”, causing that information to be empty in the resulting export.

It would be more thorough and useful if “emailCard” is included in “Card Created Date” as well.