Fix non-assignable expression issue

app/assets/javascripts/angular/directives/work_packages/work-package-column-directive.js

@@ -8,7 +8,7 @@ angular.module('openproject.workPackages.directives')
     scope: {
       workPackage: '=',
       column: '=',
-      displayType: '='
+      displayType: '@'
     },
     templateUrl: '/templates/work_packages/work_package_column.html',
     link: function(scope, element, attributes) {

app/assets/javascripts/angular/helpers/components/custom-field-helper.js

@@ -25,6 +25,7 @@ angular.module('openproject.helpers')
           return CustomFieldHelper.booleanCustomFieldValue(value);
         case 'user':
           if (users && users[value]) {
+            // try to look up users
             return users[value].name;
           } else {
             // return user id

public/templates/work_packages/work_packages_table.html

@@ -112,7 +112,7 @@
         <span work-package-column
               work-package="row.object"
               column="column"
-              display-type="column.meta_data.data_type"/>
+              display-type="{{column.meta_data.data_type}}"/>
       </td>
     </tr>