Add Folders plugin support

[wigust]

• jenkins.el (jenkins-foldername): New variable. (jenkins-jobs-view-url, jenkins-job-url): Use this.

This commit adds support for https://plugins.jenkins.io/cloudbees-folder plugin.

I tested jenkins.el still works with all 4 cases:

(setq jenkins-viewname nil)
(setq jenkins-viewname "Running")
(setq jenkins-foldername "guix")
(setq jenkins-foldername nil)