Large amount of EPUB errors

This issue is more of a note for myself to look into this further.

In the past we've had users complain about issues reading our EPUBs on certain apps but not others. See #9630 and #9062. Given that some applications have no problem we've never been able to narrow down if it's a problem with how we make the EPUB's, or the EPUB readers (possibly the file being too big for the readers to handle).

The EPUB format is a W3 standard and I found that they provide an EPUB check tool here: https://www.w3.org/publishing/epubcheck/

I ran it on the latest stable EPUB version, it checked against EPUB 3.3 rules, and it returned "887 Fatals, 12412 errors, 41 warnings, 0 infos"

Not sure when but I'll have to look into the specifics of how we're building the EPUB's more to try and fix these errors. If we can fix these and readers like apple books still have issues we can safely say it's an issue on the readers end.