osm: Convert savestate hook into a changes hook
savestate is not the right place to write wiki content, and in particular
this breaks websetup if osm's dependencies are not installed, even if
the osm plugin is not actually enabled. (Closes: #719913)
This is not a full solution: it should be possible to render the PoI files
for only the maps that changed, from the format, changes or rendered
hook. However, getting that right would require more understanding of
this plugin, and this version is enough to not break websetup. This
version is the closest correct hook to the one where this previously
took place.