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

OO-3482: add a check box to confirm the confirmation to delete some user, add more color to the confirmation popup

OO-3477: really remove initial author from repository entry, update course log with the new policy of user properties, springify export manager and co., make logging object annotation based

OO-3477 remove username/email pseudo delete, refactor userdeletemanager to use more the userdatadeletable interface, remove all data from already deleted users

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