Deprecate support for web stylesheets

[greatislander]

Prerequisites

• Can you reproduce the behavior you wish to address with all plugins except Pressbooks deactivated?
• Are you running the latest version of Pressbooks, the latest version of Pressbooks Book, the latest version of this book theme and the latest version of WordPress?
• Are you experiencing the issue while using this book theme?
• Did you perform a cursory search to see if your bug or enhancement is already listed?

Description

Due to forthcoming changes to the Book theme, Pressbooks Custom CSS will no longer be able to support web stylesheets. We need to disable inputs for web stylesheets and notify users.

Steps to Reproduce

Not applicable.

System Information

Pressbooks: 5.0-dev
Book: 2.0-dev
Custom CSS: 1.0