Support public id-based dav requests

Id-based dav requests via `.../dav/spaces/{fileId}` currently only work in an authenticated context. Requests via `.../dav/public-files` on the other hand work publicly, but only path-based, not id-based.

The Web client needs those public dav requests to work id-based as well, otherwise we would need to keep supporting ids *and* paths. That feels like an unnecessary overhead.

\cc @butonic