Add homescreen app manifest for Android Chrome

Android Chrome fixed the keyboard issue, so we can finally use the
no-URL-bar version of the homescreen app.

cache.manifest

@@ -3,9 +3,8 @@ CACHE MANIFEST
 
 # Explicitly cached 'master entries'.
 CACHE:
-sim.css
-js/
 data/
+sprites/
 
 # Resources that require the user to be online.
 NETWORK:

index.template.html

@@ -45,6 +45,7 @@
 		<link rel="stylesheet" href="//play.pokemonshowdown.com/style/jquery.slider.min.css?" />
 		<!-- meta name="mobile-web-app-capable" content="yes" / -->
 		<!-- meta name="apple-mobile-web-app-capable" content="yes" / -->
+		<link rel="manifest" href="/manifest.json" />
 		<meta name="robots" content="noindex" />
 		<meta name="google-signin-client_id" content="912270888098-jjnre816lsuhc5clj3vbcn4o2q7p4qvk.apps.googleusercontent.com">
 		<script>

manifest.json

@@ -0,0 +1,23 @@
+{
+  "short_name": "Showdown!",
+  "name": "Pokémon Showdown!",
+  "icons": [
+    {
+      "src": "favicon-16.png",
+      "type": "image/png",
+      "sizes": "16x16"
+    },
+    {
+      "src": "favicon-48.png",
+      "type": "image/png",
+      "sizes": "48x48"
+    },
+    {
+      "src": "favicon-256.png",
+      "type": "image/png",
+      "sizes": "256x256"
+    }
+  ],
+  "start_url": "/?utm_source=homescreen",
+  "display": "standalone"
+}