OO-3483: ensure that the email form in member list course element (or group) as at least one recipient

OO-3473	: delete the temporary files only at the end of the wizard and not after the first created element

OO-3472: rewrite the delete routine to delete only the file of the edited checkbox, implement a check to not delete the file if it's used by an other checkbox

OO-3469: better check variable in velocity template, check if the candidate session exists in disposing phase

OO-3459: same access rule for the export of repository entries as of courses, allow export of resource like QTI 2.1 without temporary files