Add fetchpriority attribute to main book cover image (#11444)

* Add fetchpriority attribute to main book cover image

* Fix syntax for conditional preload attribute

* Add fetchpriority to preload metatag for cover image

Apparently "Important: When preloading the image, ensure fetchpriority=high is set on the preload as well as the image."

https://developer.chrome.com/docs/performance/insights/lcp-discovery

Author: cdrini

openlibrary/templates/covers/book_cover.html

@@ -16,7 +16,7 @@
 $ srcset = '%s 2x' % (cover_lg or '/images/icons/avatar_book-lg.png')
 
 $if preload:
-    $add_metatag(tag="link", **{'rel': 'preload', 'as': 'image', 'href': src, 'imagesrcset': srcset})
+    $add_metatag(tag="link", **{'rel': 'preload', 'as': 'image', 'href': src, 'imagesrcset': srcset, 'fetchpriority': 'high'})
 
 <div class="coverMagic cover-animation">
     <div class="SRPCover bookCover" style="display: $cond(cover_url, 'block', 'none');">
@@ -34,6 +34,7 @@
                 class="cover"
                 style="$cover_aspect_ratio_str"
                 alt="$_('Cover of: %(title)s by %(authors)s', title=title, authors=author_names)"
+                $:cond(preload, 'fetchpriority="high"', '')
             >
         </a>
     </div>