Add attribution_link for impact map layers

[alexelash]

Right now, we have external_url and url, but will want to link our Attribution text to the website where we got the map layer data. This becomes especially important both to:

1. Keep the attribution text short within the application.
2. Link up to more information even if there's not a clear "About" page that works for the "About this map" link.

Refer to #1342 (comment) for some adjustments to the existing attribution text.

Once implemented, this likely will require consulting with design / pm to see what will make sense for this link.