frontmatter overhaul

Currently frontmatter for volumes, if it has a separate PDF, exists as a special-case paper type. This creates a lot of corner cases when reasoning about the mapping between authors (editors) and papers.

For example, frontmatter items are included on author pages, but not the volumes themselves, which means an editor of a volume without frontmatter PDF will not have it listed on their author page. (#7973)

Also, oddly, bibliographic metadata about the volume is generated for the frontmatter page (if there is one) but not the volume page, which simply links to a bib file for all papers in the volume.

As initially discussed in #8008, it seems like it would make more sense to

- remove frontmatter pages (redirect URLs to the volume page)
- on the volume page, link to the frontmatter PDF (if available) as a separate button
- on the volume page, include volume-level metadata
- on author pages, include volumes