Enhancement: Respect the new sharing denials capability (experimental)

The oCIS backend has added a new capability, files_sharing.enable_denials, which announces to clients if the experimental "No access" sharing role is supposed to be available. This capability is now respected by web, so that users only see the experimental "No access" role if the backend allows it.

https://github.com/owncloud/web/pull/7892
https://github.com/owncloud/ocis/pull/4903
