Refactor updating security into a helper function.

This allows us to reuse it outside of upgrade steps.
Implement WorkflowChainUpdater

For changing workflow chains without resetting existing objects to
the initial review state of the new workflow.
Refactor PlacefulWorkflowPolicyActivator to reuse WorkflowChainUpdater.

Most of the functionality was extracted from PlacefulWorkflowPolicyActivator
into WorkflowChainUpdater previously, so that it can be used standalone.
Merge pull request #31 from 4teamwork/jone-change-workflow

Implement WorkflowChainUpdater