Set default minimum zoom level in dataset map

[fostermh]

Currently when looking at the location map in a dataset, if the spatial extent is a point or very small bbox then map zoom's so far in that only ocean is visible.

Describe the solution you'd like
Set initial default zoom level so that some coastline is visible. A ckan wide default setting would work but it would be nice if this was also overridable by a setting in the dataset json.