Support RelativeTopicBase with new topics

Previously, the `TopicList`'s `RelativeTopicBase` would work incorrectly with new topics since `ITopicRepository.Load()` would load the _parent_ context's `UniqueKey`, and thus it would return content relative to the _parent_, not the new topic.

This is mitigated by temporarily creating a placeholder topic in the topic graph that can be used as the relative base. This placeholder topic must be removed from the parent's `Children` collection, as otherwise it will persist in memory, interfere with future attempts to create new children, and potentially even end up getting persisted to the database under certain circumstances (such as a recursive `Save()` triggered by an `Import()`).

Author: JeremyCaney

OnTopic.Editor.AspNetCore/Components/TopicListViewComponent.cs

@@ -94,6 +94,10 @@ public IViewComponentResult Invoke(
       if (attribute.RelativeTopicBase != null) {
         var baseTopic             = _topicRepository.Load(currentTopic.UniqueKey);
         var rootTopic             = attribute.RelativeTopicBase switch {
+        if (String.IsNullOrEmpty(currentTopic.Key)) {
+          baseTopic               = TopicFactory.Create("NewTopic", currentTopic.ContentType, baseTopic);
+          baseTopic.Parent.Children.Remove(baseTopic);
+        }
           "CurrentTopic"          => baseTopic,
           "ParentTopic"           => baseTopic.Parent,
           "GrandparentTopic"      => (Topic)baseTopic.Parent?.Parent,