Require parts and chapter numbering for books at the network level

[MichSpieler]

Feature Description

As a network manager, I want to be able to require all books to use parts and chapter numbering. Currently, this is a setting at the book level that can be changed by book administrators.

Feature Use Case

The network managers at this institution have been working with a Digital Accessibility Team to create a framework for ensuring accessibility for the books on their network. Among the requests coming out of that collaboration is the one to require part and chapter numbering for all books.

Other Notes

Link to ticket (for Support staff only)

[SteelWagstaff]

I see two potential things we could do here:

1. Create a network default to set this in new books (like we do with book themes and PDF export size)
2. Set the default value for this checkbox to on globally (https://github.com/pressbooks/pressbooks/blob/47ce282744cbc763c5131cd6e1c16f8719761fc4/inc/modules/themeoptions/class-globaloptions.php#L365)

of these, I'd have a strong preference for option 1. There may be other ways to address it as well?