Expose supported upload file formats as configurations

[giohappy]

To start the testing of GeoPackage imports from the UI the .gpkg extension must be included in the supported extensions for the client.
This option is experimental and shouldn't land in master right away. Therefore, instead of creating a dedicated client branch, we want to take the opportunity to make the formats configurable, and let the client obtain the configurations from GeoNode.

The idea is make the client get this configuration from a new property __GEONODE__CONFIG__.localSettings.geonodeSettings.upload.supportedDatasetsFileTypes (notice the additional File infix, to clarify the purpose of this settings).

GeoNode will expose this configuration inside the goenode_config template, taking it from the settings.

I'm proposing a new upload configuration level where we can gather anything related to the upload configuration. For example, the documentMaxUploadSize could be moved here too.

This change will be merged only once the backend is ready to provide such a configuration.