Book: Improve print output

Themes can override these styles by using more specific styles to override e.g. the Snap theme uses SCSS like this:

// Print style guide for book

#page-mod-book-print {

    font-size: 18px !important;

    font-family: $font-family-serif !important;

}

Test in Boost and Clean.

1. Log in as Admin.
2. Create a Course,
3. Create a Book resource.
4. Create several chapters and several subchapters.
5. Go back to the Book resource page.
6. In the Book settings cog, click on the "Print book" link.
   1. make sure sure the book print output has been improved and it inherits the current theme's css styles (ex. font style).
7. Go back to the Book resource page.
8. In the Book settings cog, click on the "Print this chapter" link.
   1. make sure sure the book chapter print output has been improved and it inherits the current theme's css styles (ex. font style).
9. Go back to the Book resource page.
10. Access one of the Book's subchapters.
11. In the Book settings cog, click on the "Print this chapter" link.
    1. make sure sure the title of the parent chapter is also displayed in the print chapter view.**