Implement simple workflow

This PR implements a simple workflow with lawgiver:

- Status "Draft" and "Published"
- Transitions "publish" and "retract"
- Roles "Visitor", "Editor" and "Administrator"
- configures `@@sharing` to only display the editor role.

Todo:
- German translations of roles

@freinhard can you take a look at the [specification.txt](https://github.com/OneGov/onegov.municipality/blob/jone-workflows/onegov/municipality/profiles/default/workflows/onegov-simple-workflow/specification.txt) ?
Should the `Editor` be able to modify / delete a published content? (I think he should)