Content Type URL signable

Redirects performed because of a content block with the type URL should be signable:

<Content type="url" href="" authz="signed" sign_owner="true" sign_viewer="false"></Content>


- A gadget with content type URL can get the current viewer and owner id (which is not possible at the moment) and use the REST or RPC API to retrieve social data.
- Better consistency in the spec regarding proxied content