Add a favicon so that browsers don't go looking in the default location

That extra request to the default /favicon.ico path on the root of the
domain, which is not a valid path because Pages has no root namespace,
was being redirected to an auth page. That auth page was setting cookies
which were then being passed along with subsequent requests, causing the
pages to not be cached. Hopefully this fixes that.

src/cache-test.html

@@ -4,6 +4,7 @@
     <meta charset="utf-8">
     <meta name="generator" content="GitLab Pages">
     <title>Cache Test | %%PROJECT_TITLE%%</title>
+    <link rel="shortcut icon" href="gitlab-favicon.png">
     <link rel="preload" href="style.css" as="style">
     <link rel="stylesheet" href="style.css">
   </head>

src/index.html

@@ -4,6 +4,7 @@
     <meta charset="utf-8">
     <meta name="generator" content="GitLab Pages">
     <title>%%PROJECT_TITLE%%</title>
+    <link rel="shortcut icon" href="gitlab-favicon.png">
     <link rel="preload" href="style.css" as="style">
     <link rel="stylesheet" href="style.css">
   </head>

src/sub/index.html

@@ -4,6 +4,7 @@
     <meta charset="utf-8">
     <meta name="generator" content="GitLab Pages">
     <title>Subdirectory | %%PROJECT_TITLE%%</title>
+    <link rel="shortcut icon" href="../gitlab-favicon.png">
     <link rel="preload" href="../style.css" as="style">
     <link rel="stylesheet" href="../style.css">
   </head>