Fixed issue #7224: comparison of dates (partial fix)

Dev: LEMval() now returns answers to date questions in yyyy-mm-dd HH:MM
Dev: format, independent of the date format that is chosen for the
question
Dev: or the particular survey. Thus, comparisons between different date
Dev: questions are possible (e.g. relevance or validation equations
Dev: "arrivaldate > departuredate") even when one question is in
Dev: dd.mm.yy and the other in mm/dd/yyyy HH:MM format
Dev: (same or previous page).
Dev: qanda_helper.php: min/max logic had to be changed accordingly
Dev: and is less complicated now.