Add icon attribute to licenses

[rufuspollock]

For licenses with an associated icon at that url to the license hash.

[Stephen-Gates]

As part of updating the csv, should I add an icon_url column and populate it where possible?

Would that break something downstream?

[rufuspollock]

@Stephen-Gates i think that is a good idea and won't break anything i believe. I would do this in a separate PR.

Also would we link to external icons or include in this repo?