admin: Use a map for home geography filter instead of textbox

[tahini]

The textbox was the easy way to get the feature in, but the nicer way would be to have a map to draw a polygon directly.

This can be done after #668 if we introduce deck.gl in the mix.

[tahini]

But being able to import a polygon from a file (to have a repeatable filter) would also be desirable, so the filter should be able to support both approaches.