Expose Mandatory setting in ContentProperty

This allows a PropertyEditor to be aware of whether it's required or not, via: `$scope.model.mandatory` (along with `$scope.model.alias`, etc)

src/Umbraco.Web/Models/ContentEditing/ContentPropertyDisplay.cs

@@ -31,5 +31,8 @@ namespace Umbraco.Web.Models.ContentEditing
 
         [DataMember(Name = "hideLabel")]
         public bool HideLabel { get; set; }
+
+        [DataMember(Name = "mandatory")]
+        public bool Mandatory { get; set; }
     }
 }

src/Umbraco.Web/Models/Mapping/ContentPropertyDisplayConverter.cs

@@ -35,6 +35,7 @@ namespace Umbraco.Web.Models.Mapping
             display.Description = originalProp.PropertyType.Description;
             display.Label = originalProp.PropertyType.Name;
             display.HideLabel = valEditor.HideLabel;
+            display.Mandatory = originalProp.PropertyType.Mandatory;
             
             if (display.PropertyEditor == null)
             {