allow addons to override the user_name for the history/log screen

git-svn-id: http://svn.krangcms.com/trunk/krang@6934 689d98c0-9023-0410-9c0b-f79783857903
allow history.user_id to be NULL so it can emptied it for legacy history entries if you like

git-svn-id: http://svn.krangcms.com/trunk/krang@6935 689d98c0-9023-0410-9c0b-f79783857903