Searchkit: Download spreadsheet should use standard date format, not the format set in Date Formats

If you're downloading a CSV or other spreadsheet from Searchkit results or a SK Table, you'll get dates formatted per the Date Display format setting, which by default is August 9th, 2012 1:17 PM. That's a format that takes some wrangling to get Excel or other spreadsheet programs to recognize as a date.

Instead, the date should be exported as it is using export from non-SK results, in standard format 2012-08-09 13:17.

Related issue on data types in ods or xlsx files.

Tested on dmaster (5.58).