Inject base path into manifest

Might fix #1181, although not for the reasons I initially
thought (because the URLs are resolved from the manifest path, not the
path of the current page). This should ensure that the URLs used by the
manifest are always correct regardless of the manifest's path.

src/browser/media/manifest.json

@@ -1,12 +1,12 @@
 {
   "name": "code-server",
   "short_name": "code-server",
-  "start_url": "../../../..",
+  "start_url": "{{BASE}}",
   "display": "fullscreen",
   "background-color": "#fff",
   "description": "Run editors on a remote server.",
   "icons": [{
-    "src": "./code-server.png",
+    "src": "{{BASE}}/static-{{COMMIT}}/src/browser/media/code-server.png",
     "sizes": "384x384",
     "type": "image/png"
   }]