Fix 0010922: Prevent jscalendar from being imported more than once

jscalendar doesn't work if it's included more than once (recursion error
according to Firefox's Error Console). The date_print_calendar()
function always used to include the javascript each time it was called.
However this function can be called multiple times on a page load and it
therefore shouldn't include the same javascript/css files each time.
This function has been changed to import the javascript/css files only
on the first instance.