Set start_url in manifest.json correctly

[damooo]

Hello guys,

In manifest currently start_url is setted to / at below:

sapper-template/static/manifest.json

Line 7 in 7a95820

"start_url": "/",

But when apps is mounted at custom mount_path, absolute root is not the start_url. Thus it will not be recognized as pwa, as service-worker is not scoped to absolute root.

If it is setted to ./ , it is app root, instead of absolute root, as we are serving manifest.json from app root.

We can also set scope to same value by default.