Support GeoPackage and GeoJSON uploads

[abyrd]

Shapefiles are completely archaic and we've wasted a lot of time diagnosing and dealing with Shapefile-specific issues. The use of sidecar files, 10-character property names with no awareness of UTF-8, etc. are ridiculous in 2021.

We should be part of the movement to eliminate this mess, by making it possible for our users to use other formats and encouraging them to do so in the documentation. An informal poll cited on the "Shapefile must die" page finds that the most favored replacements are GeoPackage and GeoJSON, which are supported by almost 100% of the voters.

See:
http://switchfromshapefile.org
http://www.geopackage.org