Author: David Mudrák
MDL-48165 workshop: Validate the rubric editing form
http://git.moodle.org/gw?p=moodle.git;a=commit;h=fa36f1d1ab78605442d2e2730f78aacb80f52c2f
http://github.com/moodle/moodle/commit/fa36f1d1ab78605442d2e2730f78aacb80f52c2f
+ 2.8 en [mustbeunique,workshopform_rubric]