Feature/enable chapter nav for drafts

[william-man]

Description of changes

This PR changes ChapterNavigation->getItems to also get draft posts for users that have the edit_posts capability so that drafts that are in preview can show their full chapter/page navigation of all posts related to each other.

To test:

1. Fetch this branch feature/enable-chapter-nav-for-drafts.
2. In admin create some users that can edit posts and optionally a subscriber.
3. Create some draft posts and set a parent for them. Use the Preview link in the editor to view the draft and check that it displays all chapters in the page nav as a user who can edit posts.
4. Publish a post, the post should still display all posts.
5. Switch to a user that can't edit posts or log out of current user and check the published post again. It should only show chapters of published posts.

Checklist

• Changelog updated
• If new release: major version tag to be bumped after release (see docs)

[sbrody]

Works well locally