Ensure TopicReferenceAttributeViewComponent honors current key

The `TopicReferenceAttributeViewComponent` previously had a hard-coded value of `TopicID`. While it is indeed indended primarily to support derived topics (i.e., `TopicID`), it is possible that other attributes may use it in the future if they have similar requirements (i.e., a topic reference that triggers a postback). As such, this maintains the existing functionality while potentially providing more flexibility in the future, if needed.

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Areas/Editor/Views/Editor/Components/TopicReference/Default.cshtml

@@ -2,7 +2,7 @@
 
 @{
   var attributeDescriptor       = new TokenizedTopicListAttributeTopicViewModel() {
-    Key                         = "TopicID",
+    Key                         = Model.AttributeDescriptor.Key,
     ContentType                 = "TokenizedTopicListAttribute",
     Description                 = Model.AttributeDescriptor.Description,
     Title                       = Model.AttributeDescriptor.Title,