Subtitles and chapter authors given same bookmark level as chapter titles in PDF format

[mcgratay]

Description

Pressbooks creates tagged PDFs, which allows readers to utilize the "Bookmarks"/"Table of Contents" feature on their chosen PDF viewer to navigate the file. The post subtitle and post author elements appear tagged at the same level of hierarchy as post titles.

These elements should not appear in the bookmark list at all. First image is from Preview. Only post should be shown in hierarchy beneath chapters. Second is from Adobe Acrobat Reader.

Example: https://accessiblemarkup.textopress.com

Steps to Reproduce

1. Create a new chapter
2. Scroll down to the Chapter Metadata section beneath the editor
3. Enter a subtitle in the "Chapter Subtitle" text field
4. Select an author in the "Chapter Athor" field (note: must be an existing contributor)
5. Save the chapter and export to PDF

Expected behavior: Bookmarks for PDF show an accurate representation of content in the book

Actual behavior: Chapter metadata elements are shown in equal hierarchy to chapter titles

[SteelWagstaff]

• [ ] Step 1: fix underlying XHTML (replace improperly used headings with semantically appropriate elements, i.e. divs/spans)
• [ ] Step 2: test exports to make sure ToC is fixed
• [ ] Step 3: check CSS for webbook/PDF to make sure appearance has not been degraded (i.e. the relevant CSS targets the class and not the elements changed in step 1)
• [ ] Step 4: check impact on non-Buckram themes