Poll the absolut path of the manifest.json file (#120)

Instead of using a relative path to manifest.json use an absolut path.
This makes sure that we detect that Home Assistant Core is available
also on our Apps, which navigate to `/auth/authorize?...` by default.
The relative path then polls the wrong location `/auth/manifest.json`.

With the absolut path we make sure that the correct location is polled
on our Apps as well.

rootfs/usr/share/www/static/scripts.js

@@ -61,7 +61,7 @@ self.fetch ||
   });
 
 function testAvailable() {
-  fetch("./manifest.json").then(function (res) {
+  fetch("/manifest.json").then(function (res) {
     if (res.ok) {
       location.reload();
     } else {