Option to edit default background image for pages on root network

[lgumb]

Feature Description

It would be great if network managers had the ability to change/customize the background image for pages on the root network. Currently, we can only change header images on the network homepage.

Feature Use Case

Currently, network managers are unable to change the background image for pages on the root network without using custom CSS. I can imagine that many super admins for their university PressbooksEDU networks are librarians like myself and without any CSS skills. This would be really helpful to those of us wanting to change the background image. Thanks!

[SteelWagstaff]

I propose that we do two things:

1. Add a customizer option for Aldine that allows network managers to use a header image which would replace https://github.com/pressbooks/pressbooks-aldine/blob/dev/dist/images/catalog-header.jpg for the catalog and other child (i.e. not-main/home) pages on the root site (in much the same way that Header Image replaces https://github.com/pressbooks/pressbooks-aldine/blob/dev/dist/images/header.jpg if it has been set:

   pressbooks-aldine/header.php

   Lines 47 to 54 in 743e20f

   	if ( is_front_page() ) {
   	if ( has_header_image() ) {
   	echo( get_header_image() );
   	} else {
   	echo get_template_directory_uri() . '/dist/images/header.jpg';
   	}
   	} else {
   	echo get_template_directory_uri() . '/dist/images/catalog-header.jpg';

   ). When doing this we may wish to change the name of the current 'Header Image' option to say 'Home Page Header Image' and this second option to say 'Child Page Header Image' or something to that effect.

2. On the edit page menu for individual pages on the site root we should add a small button that allows the user to set a custom header image for that page. It would look/function much like the featured image function for core WordPress.
   

When a page has a custom header/featured image, that image would display as the page's header background image instead of the theme default.

[SteelWagstaff]

See also #189