Use current date when saving mail drafts in case subject is empty

Thanks to Karel Volný for reporting this. Previously, a message with an empty
subject would produced file ".draft" which did not match the GUI's wildcard and
therefore was not visible in the file picker when resuming these drafts.

Thanks to Thomas Lübking for pointing out that some characters are best avoided
here.

BUG: 324109
REVIEW: 112309