preconnect: add crossorigin (#450)

It seems it's needed for these domains

src/layouts/partials/head.html

@@ -1,8 +1,8 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1">
 
-<link rel="preconnect" href="https://github.com">
-<link rel="preconnect" href="https://avatars.githubusercontent.com">
+<link rel="preconnect" href="https://github.com" crossorigin>
+<link rel="preconnect" href="https://avatars.githubusercontent.com" crossorigin>
 <link rel="preload" href="/assets/brand/bootstrap-logo-shadow.png" imagesrcset="/assets/brand/bootstrap-logo-shadow.png, /assets/brand/bootstrap-logo-shadow@2x.png 2x" as="image">
 
 <link rel="profile" href="https://gmpg.org/xfn/11">