Use credentials to request manifest.json file

This is needed to properly fetch the manifest.json
file when Gramps runs behind an authenticating
proxy like Pomerium or Cloudflare Zero Trust, as
documented here: https://web.dev/articles/add-manifest#link-manifest

index.html

@@ -9,7 +9,7 @@
   <link rel="stylesheet" href="fonts/fonts.css">
   <link rel="stylesheet" href="tippy.css">
   <link rel="icon" href="images/favicon.ico">
-  <link rel="manifest" href="manifest.json">
+  <link rel="manifest" href="manifest.json" crossorigin="use-credentials">
   <meta name="theme-color" content="#6D4C41">
   <meta name="mobile-web-app-capable" content="yes">
   <meta name="application-name" content="Gramps">