Define communication and action triggers for workflow parts

[sdruskat]

In #75 (comment), we have defined different workflow parts.
At 3 points in the whole workflow, we have communication needs (messaging), and need for triggers:

1. User starts workflow in Preparation part.
2. User confirms metadataset at the end of Collation.
3. Curator confirms publication-readiness at the end of Curation.

We need to weave this

1. into the CLI implementation
   • e.g., do the four target "steps" need to be redefined?)
2. into the CI "implementation"
   • e.g., how do we facilitate messaging and action triggers (e.g., manual triggers, tags, commit messages, etc.),
   • how do we pass data between the different workflow parts in CI (REST APIs, caches, artifacts)

It would be good to have a BPMN view on this, @poikilotherm to the rescue.