Add georeferencing widgets: draw dataset extent on map, or select pre-defined extent

This feature request concerns ckanext-spatial, ckanext-scheming and maybe it's not CKAN-core enough to get traction here - please advise if this is the wrong place to ask.

In the spatial widget, let users either pick a pre-defined spatial extent (e.g. administrative boundaries), or draw the extent directly (as rectangle or polygon), in addition to the good old paste-valid-GeoJSON which non-spatial users tend to struggle with.

Here's a tentative implementation in action: https://vimeo.com/116324887

If you'd like to have a go for yourself, request a login to http://data-demo.dpaw.wa.gov.au/

See also https://github.com/ckan/ckanext-spatial/pull/93 and https://github.com/open-data/ckanext-scheming/issues/11 (with backstory and use cases).

Start by reviewing the ckanext-spatial spatial widget and the linked ckanext-spatial pull request #93, then read ckanext-scheming issue #11 for the use cases and backstory. Done should let users choose a pre-defined extent or draw a rectangle or polygon, while retaining valid GeoJSON input.

Written by the indexing model from the issue text.