Use pb_cover_large thumbnail image for API

[arzola]

This PR #2174 added large thumbnail URLs to the API. We want to change

pressbooks/inc/datacollector/class-book.php

Line 474 in 2fa944f

$cover_path = wp_get_attachment_image_url( $cover_id, 'large', false );

to pb_cover_large

[arzola]

@SteelWagstaff medium_large is returning a bigger thumbnail than large hahaha, maybe we could use pb_cover_large size because it seems we are overriding wp image sizes here https://github.com/pressbooks/pressbooks/blob/2fa944f49a20d531e3e6dcdb4e90635ac7c505f3/inc/image/namespace.php pb_cover_large is the one that is being used on the theme to show the book cover. I think is good enough it looks good on my retina display