Modernize CI via Jenkinsfiles (part 1)

The Jenkins-based CI currently relies on old-fashioned scripts that must be edited in the Jenkins web UI or copy pasted back and forth from an editor. This is unwieldy and changes to the scripts are not traceable.

This task is about migrating the scripts used to Jenkinsfile so that they can be versioned along the code. This also has the benefit that testing modified jobs can be more easily done from the CLI via the jenkinscli tool.