Feature request: spatial widget

Ok this is stretching it a bit, but since scheming tackles custom form widgets, and ckanext-spatial provides this really good to have but (for end users) quite challenging `spatial` field, it would be very useful to integrate both.

A form_snippet spatial.html should allow users to
- paste a valid GeoJSON geometry as per ckanext-spatial, or
- [draw a polygon](http://leafletjs.com/2013/02/20/guest-post-draw.html) on a leaflet map, writing back the geojson geometry of the convex hull (if several polygons are drawn) or
- upload a geojson file with the bounding polygon (or a bunch of features), then save the convex hull's geometry

A display_snippet spatial.html could render the dataset extent just like ckanext-spatial does as a map, hiding the unhelpful WKT blurb. Edit: [done](https://github.com/florianm/ckanext-scheming/tree/11-spatial)

Also posted to [ckanext-spatial](https://github.com/ckan/ckanext-spatial/issues/91).

The request names form_snippet spatial.html and display_snippet spatial.html; start by reviewing the existing ckanext-spatial behavior and the linked 11-spatial branch. Check the Leaflet drawing reference and confirm that GeoJSON input, upload, convex-hull handling, and map display all match the requested behavior.

Written by the indexing model from the issue text.