Allow reordering, even if `IsProtected` or `DisableDelete`

[JeremyCaney]

Currently, if a topic is marked IsProtected (#35) or its ContentTypeDescriptor is marked as DisableDelete, then the topic is marked as draggable: false in the JSON data, thus preventing the topic from being moved. This is by design. Ideally, however, this should not prevent the topic from being reordered within the same parent, as this doesn't affect e.g. the path to the topic.

[JeremyCaney]

To support this, I renamed the property on the QueryResultTopicViewModel from IsDraggable (draggable) to IsMovable (movable) (5a1fcf6), and then modified the validation logic within the DraggableTreeView JavaScript component to use the nnodedragover event instead of the startdrag event, as that gives us access to both the source node (dropNode) as well as the target node (target) (d5d1730). This was merged into develop as part of bd9de01.