Improve template API calls prefetching in the email editor

When we don't know template we prefetch recent emails for recent emails
tab to open faster, and in case the template is known, we prefetch the template.
[MAILPOET-6370]

mailpoet/lib/EmailEditor/Integrations/MailPoet/EditorPageRenderer.php

@@ -214,9 +214,14 @@ class EditorPageRenderer {
       '/wp/v2/settings',
       '/wp/v2/types?context=view',
       '/wp/v2/taxonomies?context=view',
-      '/wp/v2/templates/lookup?slug=' . $templateSlug,
     ];
 
+    if ($templateSlug) {
+      $routes[] = '/wp/v2/templates/lookup?slug=' . $templateSlug;
+    } else {
+      $routes[] = '/wp/v2/mailpoet_email?context=edit&per_page=30&status=publish,sent';
+    }
+
     // Preload the data for the specified routes
     $preloadData = array_reduce(
       $routes,