Author: Shamim Rezaie
MDL-71410 mod_assign: implement activity_dates for the assignment module
http://git.moodle.org/gw?p=moodle.git;a=commit;h=b700f690abaec7d0228cec091398cfa7c7a9830a
http://github.com/moodle/moodle/commit/b700f690abaec7d0228cec091398cfa7c7a9830a
+ 3.11 en [activitydate:submissionsdue,mod_assign]
+ 3.11 en [activitydate:submissionsopen,mod_assign]
+ 3.11 en [activitydate:submissionsopened,mod_assign]