Better support for moving folders and jobs in folders

Implements [JENKINS-20008](https://issues.jenkins-ci.org/browse/JENKINS-20008), [JENKINS-18028](https://issues.jenkins-ci.org/browse/JENKINS-18028), and [JENKINS-18680](https://issues.jenkins-ci.org/browse/JENKINS-18680). Counterpart is https://github.com/jenkinsci/cloudbees-folder-plugin/issues/2.