Bug fixes & tiny improvements

Fixed bug where class statistics would not allow selecting existing populations & removed the earlier temporary fix for this which disabled the validation completely.

Changed the copy student numbers -functionality in Students of population > General tab: removed space from the list, now only separated by doublecomma.

Changed completed courses search -functionalitys way of parsing studentnumbers, now it accepts doublecomma as separator also.

Internal fixes to logging in updater, and fixed bug where updater would crash hourly.