Crib some css from VE for empty external links

Utilizes a css counter to populate external link anchors
as [1], [2], etc. This css is essentially lifted from VE.

Bug: 57451
Change-Id: I94f9d75bf82e4a930eded3f3c0216db7dda3970c
Merge "Crib some css from VE for empty external links"