Use updated steps to complete a workflow after fixing an error

Jeff_182127

Right now, when an instance of a workflow is run, it runs all of the steps that were there when the instance was first started. Oftentimes, the fix entails revising a step of that workflow, but it won't apply to the existing run. I would love to have the ability to complete that instance using the updated steps without having to start a new instance (in addition to the current process of completing the run with the original steps).

This is related to, but a little different than, another idea posted by @David_931366 here: https://community.pushbot.com/discussion/1587/retrigger-an-instance-after-a-failure-retaining-original-data#latest