Workflow notifications, e.g. Slack/Email when workflow finishes

Integrate the notification library that @alexmt developed so that we can notify users about the state of their workflow.

Start by locating the notification library developed by @alexmt and the workflow state or completion entry points. Trace how workflow state is exposed to users and confirm the intended Slack and email notification behavior; done means users can be notified when a workflow finishes.

Written by the indexing model from the issue text.