Implement WAC-Allow header

Currently we find about the rights we have on a resource by fetching the whole ACL resource (and we don't use the `Link` header, which make the code incompatible with Solid).

But there exists a header to know about the rights on the resource !

http://solid.github.io/web-access-control-spec/#wac-allow

This means we would only need to GET the resource, and we could know faster if we have write permission, for example.

> The data provider should still fetch the ACL resource if no WAC-Allow header is provided.

No files, tests, or entry points are named. Locate the code path that fetches the full ACL resource and read how resource permissions are checked; implement handling for WAC-Allow on the resource response, while preserving the ACL fetch when that header is absent, then verify write-permission checks.

Written by the indexing model from the issue text.