Better support for moving folders and jobs in folders

So far implemented [JENKINS-20008](https://issues.jenkins-ci.org/browse/JENKINS-20008). Need to also implement [JENKINS-18028](https://issues.jenkins-ci.org/browse/JENKINS-18028) and [JENKINS-18680](https://issues.jenkins-ci.org/browse/JENKINS-18680), and probably add a _Move_ action to `cloudbees-folder` (retaining the option for a more sophisticated implementation in the closed-source `cloudbees-folders-plus`). Counterpart is the [folder-moving branch in cloudbees-folder-plugin](https://github.com/jenkinsci/cloudbees-folder-plugin/compare/folder-moving).