User should not be asked for grant authorization if already done

Each time a user uses the OAuth2 authorization for the same application, Nuxeo always asks him for grant authorization. If the user already grants the authorization, Nuxeo should not ask him again while there is a valid token Nuxeo side.

Moreover, the administrator should be able to set an "auto-grant" flag when registering an OAuth2 client to completely avoid the grant access process: this makes sense for the Nuxeo Mobile application for instance.