Custom chloropleth maps

[elipollak]

I was looking into adding custom maps support for Kenya (we love Redash over at Apollo Agriculture) but it seems like there may be a preference for a general solution like #4637 over specific additions like #4637

Is there a strong preference one way or another? I'd love to add Kenya and can follow all of your lead on which approach you would prefer to adding

[elipollak]

I added custom maps support here #7639 but let me know if a different conceptual approach is preferred (i.e. just adding the new maps directly)