Author: Petr Škoda
MDL-34853 use new duration setting type in enrol_self
http://git.moodle.org/gw?p=moodle.git;a=commit;h=0bf660cad6deae3162b021a5b9d8e057290b6d2f
http://github.com/moodle/moodle/commit/0bf660cad6deae3162b021a5b9d8e057290b6d2f
+ 2.4 en [enrolperiod_desc,enrol_self]