Automatically create a node/% layout when one is created for node/NID

I was thinking that when someone did create a layout by typing in entity/numeric, we could use a submit handler to switch it to the expected use: a layout at node/% with a visibility condition limiting it to the single entity, either by ID or path.

That way nobody needs to learn a new behavior, but we deliver the feature we intended, and it will work closer to how they expect.

This is a follow-up to both https://github.com/backdrop/backdrop-issues/issues/2636 and https://github.com/backdrop/backdrop-issues/issues/5433#issuecomment-100...

From @docwilmot:

What if we automatically fixed this for the user? Meaning if a user types in entity/numeric we automatically converted this to entity/% and added a condition for entity id = numeric ?

from @jenlampton:

Can we add a condition for path instead of entity ID? I feel like that's going to be more in line with what people expect to happen.

We should also add a message saying "A condition has been added for this layout to match the path you entered" so that people don't think it's a bug.