Use credentials to request manifest.json file (#510)

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">