create user with same name as ckan.site_id at startup

At the moment a sysadmin user (with the same name as ckan.site_id) is created by logic.action.get.get_site_user (the first time it is called). This is needed by some logic functions and extensions in particular. There is a potential bug where a user could create a user with this name (and break things) before this function is called as it is not called at startup.