Author: Zig Tan
MDL-38301 mod_assign: Fix date validation logic & message consistency
http://git.moodle.org/gw?p=moodle.git;a=commit;h=088f6b973a5314fd06c5084d795dc9a4ded354d1
http://github.com/moodle/moodle/commit/088f6b973a5314fd06c5084d795dc9a4ded354d1
+ 3.5 en [cutoffdatevalidation,mod_assign]
+ 3.5 en [cutoffdatefromdatevalidation,mod_assign]
+ 3.5 en [duedatevalidation,mod_assign]