* (bug 21196) Article::getContributors() no longer fail on PostgreSQL

* (bug 21196) Article::getContributors() no longer fail on PostgreSQL

Changed User::loadFromRow() to allow partial row (user_id, user_name, user_real_name can be set independently from other fields) userand removed User::$mDataLoaded, replaced by User::$mLoadedItems (marked as private) that can be an array with already loaded items or true when all data has been loaded. Changed GlobalFunctions.php and Database.php accordingly, no use of User::$mDataLoaded in extensions.