make it clear that choosing taxonomy queue vocab is a one-time choice

Issue Summary

After creating a taxonomy subqueue, I can't change the vocab.
This is probably obvious if I stop to think about the underlying architecture of the module, but with my user hat on I don't want to think about that.

So this text needs to change: "Select which vocabularies to use; each unique combination of terms from all of these vocabularies will have a subqueue."
- When creating the queue, make it clear that this is a choice I can't change later
- When editing the queue, don't tell me about something I can select when I clearly can't! ;)