Feature/support preview links for page breaks

[RobjS]

Description of changes

This PR adds support for previewing a draft page-break based long read.

Before, the chapter navigation, and pagination links, would not render correctly. Now, they will.

It also does some minor refactoring to make the PageBreakChapterNavigation class more readable.

How to test

1. Spin up a site that uses this plugin (e.g. UKSS), and checkout this branch
2. Ensure you have page-break based long reads activated, by setting the DXW_PAGE_BREAK_LONG_READ constant to true.
3. Create a long-read item that uses page breaks, and save it as draft
4. Preview the long read, and confirm that chapter navigation and pagination links function correctly

Checklist

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

[william-man]

confirm, the chapter navigation and page links works while post is in the preview