Add publicstorage middleware

[butonic]

With #2646 listing a folder by id will show the permissions of the owner. While the public scope can prevent writes when tha public link is read only, there is currently no way to reduce the permissions to reflect the link permissions.

The publicstorageprovider does roduce permissions correctly when accessing files with a reference that uses the token as the root and a relative path.

When using an id based reference the request is routed to the real storage provider, not the publicstorageprovider.

As a consequence we need a middleware for nomal storage spaces (or in the gateway) that reduces permissions when a public scope is in the context.