Support the Jenkins 'read-only' feature

[boydj]

Your checklist for this issue

🚨 Please review the guidelines for contributing to this repository.

• Link to any upstream changes that might be required (for example Jenkins Core pull request)

Feature Request

Jenkins has released the 'read-only' feature for preview, which allows for restricting configuration UI and API. Update the Jelly templates in JTE so that JTE is compatible with read-only Jenkins.

https://www.jenkins.io/blog/2020/05/25/read-only-jenkins-announcement/

https://www.jenkins.io/doc/developer/views/read-only/

[steven-terrana]

Thanks for the feature request!

That makes sense and aligns with JTE's goal of helping provide governance to Jenkins pipelines.