Port to libsoup3 API

libsoup2 is deprecated and unfortunately, while libsoup2 and libsoup3 can be installed in parallel, they can't be loaded within the same process - which causes problems if the other is loaded transitively by another dependency of some software.

Needs a port to libsoup3 API.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information