Only allow specific content types to be top level book pages

Problem/Motivation

Currently there is no way to have a specific content type for top level books. Often, the top level of a book displays differently and has a different purpose. I would like to allow basic pages to be top level and allow things like events, projects, etc under that. When authors can make any content type the top level, it's difficult to account for the theming of different layouts as a top level book.

Proposed resolution

• Add a setting "Content types that can be top level book pages"
• Remove the "- Create a new book -" option on the node edit screen for content types not allowed