ProjectAuthenticator -> QueueItemAuthenticator

As Jesse pointed out, contextual information is often useful in authenticating the build, which means we need to take Queue.Item (initially AbstractBuild was the parameter, which provided the context, and I failed to accommodate that in transition to AbstractProject.)

To still allow Queue.Tasks to provide a meaningful value fallback to Queue.Task.getDefaultAuthentication() as opposed to hard code it to ACL.SYSTEM. This allow plugins like remote-terminal-access to supply a meaningful secure value without forcing a configuration change to the user.