Make url and label mutually exclusive

[jgraham]

If a links entry has both url and label fields it's unclear exactly what that means: is it just a compression of two separate entries, one with url and one with label, or does it mean that the URL is a link for the label itself in some way?

I think it's supposed to be the former, and therefore for simplicity we should make url and label mutually exclusive.

[foolip]

@KyleJu can we lint for this?