Add a "notes pane" to the edit screen

User needs:
- a convenient place to see what's happened with a post over time, in context

As an MVP of basic discussion on PRs, add a panel to the side of the edit screen to house information.

For now, just list the major actions. See: https://phae.github.io/netlify-sketch/review.html (the coloured boxes in particular showing status changes) Commenting etc. can go in a seperate issue.

Acceptance criteria:
- [ ] User can click a link marked "Notes" from a tab menu above the preview pane.
- [ ] In most-recent-first order, a user can see who created the post first, who reviewed it and who published it.

Start by locating the edit screen and its tab menu above the preview pane, then review the linked sketch for the intended Notes layout and status-change boxes. Confirm how post creation, review, and publication events are represented before implementing the most-recent-first list; done means the Notes link opens a panel showing those three actions and their creators.

Written by the indexing model from the issue text.