Use TURF Properties files for site description sidecar files.

Now that the more concise TURF Properties files are available via URF-85, switch to using them for TURF generated site description sidecar files.

In addition improve the format of the sidecar file name to use the form filename.ext._.turf-properties rather than filename.ext.@.turf, because @ is used in TURF for dates and also in Guise Mummy for posts (also based on dates). Here the _ is a placeholder for "some subject being described", and if this works well then we can consider using this approach in other circumstances.