Add not georeferenceable option to preparation stage

[BNAndras]

https://oldinsurancemaps.net/resource/328 is an example of a sheet that isn't a map. Currently, this sheet would sit in the unprepared area, but that's a difference between a page not yet reviewed but can be prepared and one that is reviewed but can't be prepared. Setting a sheet as not georeferenceable would set it aside from the usual georeferencing workflow but still allow it to be tracked as part of a volume.