Implement "Duplicate" feature for Jobs

We need to store and display a history of job details, but we'll do this in two ways. Firstly, to track small revisions and corrections to existing jobs/contracts, we'll use CiviCRM's built-in logging feature. Secondly, to track major changes/promotions/contract-renegotiations/etc, we'll include a "copy/duplicate" feature that can create a new job (using an existing job as a template).