IIIF URL generation in Postgres

Try to generate IIIF URLs for zones, maybe with a method similar to the one for ListProcessElements.

The goal is to not have to fetch polygons at all, as Django's PostGIS bindings are known to be slow (#460 (closed)). If generating the entire URL is not possible, we can at the very least:

• Get a bounding box for the polygon, which reduces everything to two points?
• Get only the crop portion of the IIIF URL, then just do some concatenating in Python?