ui: fix assign text & core: allow owner get homework detail (#345)

packages/ui-default/templates/problem_edit.html

@@ -43,7 +43,7 @@
                row:false,
                columns:3,
                label:'Assign',
-               help_text:'Assigned groups',
+               placeholder:_('Group / UID'),
                name:'assign',
                value:pdoc.assign|default([])|join(', ')
              }) }}