minor: removed nested_urls in favour of simpler system

@balessan these changes fix the étuc bug you reported to me in chat

took it as an opportunity to simplify things a bit. Tested with the Hubl test1 app, could you also test with the étuc front-end please?

I think minor is the correct version increase? Shouldn't affect any production setups as far as I've checked but as it removes nested_urls I think it should be? Is there a better way we could be notifying application devs to test breaking changes? Because I hope to refactor the LDPPermissions system soon as well 😅 (#305 (closed) )

Let me know and I'll prepare some release notes