EZP-23089: REST siteaccess API

> Status: RFC
> Issue: http://jira.ez.no/browse/EZP-23089

Simple approach: a unique resource lists siteaccess the user can login to.
not too clear if uriPrefix will always include protocol + hostname or not (then, would it be only when any of the 2 is different from currently used ones? When implementing this, take into account ssl-terminating proxies ;-) )