Allow to access to XWikiPreferences properties in REST with guest

Right now the XWiki.XWikiPreferences page has view rights only for Admin users: this means we cannot access its property values using a REST call.

This is limiting in cases where we need to know some property values in our Javascript: right now we rely on script services with velocity to access such values, but it leads to various bugs because of the browser cache.

I suggest that we create a whitelist of properties that are allowed to be accessed through REST calls in order to be able to use in the frontend.

Forum proposal opened in: https://forum.xwiki.org/t/allow-to-access-to-xwikipreferences-properties-in-rest-with-guest/6052/