Update maps.stamen.com Watercolor links to point to Smithsonian version

[almccon]

As we did for Maperture in stamen/maperture#221 we should have the maps.stamen.com version of Watercolor point to the Smithsonian version. We'll also need to update the attribution text for Watercolor at the bottom of the maps.stamen.com page.

[almccon]

Note, this isn't quite as easy as in stamen/maperture#221, because in this repo we have to change both the index.html and watercolor/index.html, and both of those places still (surprisingly!) use tile.stamen.js so we might need to change that script too and add special case logic for Watercolor.

[kelsey-taylor]

reassigning this one to you @almccon 🙃