Add something like `is_leaf` Boolean field to serialized ContentNodes to replace common "leaf or branch" checks in JS code

In the ContentNodeViewSet (and other related viewsets), it would be nice to have a field like is_leaf that returns true whenever the node is a resource/leaf and false if it is a topic or a channel.

There are dozens of places in the JS code where we have to check if this is the case based on the kind field that could be simplified if we had this field.

[rtibbles]

Noting that on the backend TOPIC should be the kind on every node that isn’t a leaf node (even the channel root node), wherever the special CHANNEL kind is being set it is only on the frontend.

So, the annotated field on the ContentNodeViewset would just be a simple boolean check of kind == 'TOPIC'.

[jonboiser]

So, after adding the is_leaf field on the ContentNodeViewset, checks on the frontend like kind === ContentNodeKinds.TOPIC || kind === ContentNodeKinds.CHANNEL can be replace by a simple !is_leaf. Am I correct?

Yes, that’s right. I think we can even safely replace the kind === ContentNodeKinds.TOPIC check (without the check for CHANNEL) in most places as well.