WebSearch: optimise can-see-hidden-MARC-tags check

• Optimises can-see-hidden-MARC-tags check. Before, the check was done by calling RBAC engine on-the-fly for every record. This patch moves the check onto login time and stores the result in user_info object so that it is rapidly reused at search time without further querying of the database.
  
  This greatly reduces the number of SQL queries on the search results pages. For an example search page, the number of SQL queries went down from about 360 to about 230.