Ticket 5: Shipped items derive Changelog Entries plus feed

## Parent

Part of screen-gd/Closure#1.

## What to build

Moving a Roadmap Item to Shipped produces an editable draft Changelog Entry, published to a newest-first changelog feed.

## Acceptance criteria

- [ ] Shipping an item derives a draft Changelog Entry without double entry
- [ ] Maker can edit the entry before publishing
- [ ] Feed reads newest-first

## Blocked by

- screen-gd/Closure#3