Make polygon simplification easier to use

[Permafacture]

Since polygons will always be in WGS84 spatial reference, we should be able to specify a a decent area threshold at the Map level. A static number would not work well as a default for high and low latitudes. Maybe each polygon should interpret the map level area threshold based on it's average latitude. There's not much we can do for single polygons that cover a large range of latitudes at this point though.