Use asset helper for webmanifest icons

Specifying a host is optional, since browsers will resolve the path anyway. However, the absolute path entered didn't work with another relative_url_root. We could either fix it by removing the leading / (making the icon relative to the webmanifest) or simply by using an asset helper.

app/views/pwa/manifest.webmanifest.jbuilder

@@ -15,12 +15,12 @@ switch_locale do
   json.icons do
     json.array!([
       {
-        src: '/icon.png',
+        src: asset_url('icon.png', skip_pipeline: true),
         sizes: '512x512',
         type: 'image/png',
       },
       {
-        src: '/icon.svg',
+        src: asset_url('icon.svg', skip_pipeline: true),
         sizes: 'any',
         type: 'image/svg+xml',
       },