administror (admin) user role and site mode

[jmvanel]

In addition to current site mode, where every connected user can add anything, we will have a "admin" site mode, where the services in
https://github.com/jmvanel/semantic_forms/blob/master/scala/forms_play/conf/routes
and not in
https://github.com/jmvanel/semantic_forms/blob/master/scala/forms_play/conf/nologin.routes
will be accessible only to admin.

This is the first step for managing as requested the site
https://data.grottocenter.org/
which currently works under the nologin mode ( immutable content except LOD content added by /display )

The next steps will be:

1. new user type, content managers, that will be able to /create and /edit URI's, and /load RDF content by HTTP POST;
   this avoids having several real persons acting as admin
2. admin can add content managers
3. CMS mode: every connected user can propose content, which is only visible to her and content managers, and will be approved by content managers;
   this additional facility is to replace the use case where a contributor sends by mail to a content manager an RDF document prepared by an external RDF editor, for example a non restricted SF site, or a localhost installation of SF.