Plugin preview for #48

Built from 6244cf8. This preview's registry lists every RustGIS plugin, with this pull request's plugins served from here:

Open in RustGIS

It opens web.rustgis.app's Plugin Manager on this preview's registry. Install a plugin there; RustGIS shows its permissions first. The registry is only used for that visit. On the desktop, set Plugins › Plugin Manager › Settings › Plugin registry to registry.json here.

This is unreviewed code from a pull request. Install it only if you are reviewing that pull request.