Know when a workflow is in read-only mode

[taylordowns2000]

Sometimes the currently workflow isn't editable. (Most likely because the user is viewing an old "snapshot" of that workflow.) Apollo, bless it, still tries and really does think that it's capable of editing. The Lightning front-end is graceful, but not particularly forthcoming. Here, we see Apollo thinking it's added a step and Lightning simply ignoring it.

We should update Apollo to expect a "readOnly" flag (huh... i wonder if "advice mode" should be a thing we consider) so it knows that even when it proposes YAML or Javascript, the changes cannot and will not be applied.

cc @elias-ba , @hanna-paasivirta , @josephjclark