Fix app manifest `redirects` property support.

The webapps actors isn't synced with webapps.jsm new feature.
We are (at least) missing `redirect` property support when installed a privileged app.
https://developer.mozilla.org/en-US/docs/Web/Apps/Manifest#redirects

Here is a simple fix for the simulator, but we should upstream this fix (and the other features we have added like receipts) and use the upstream version!

I also identified similar issue for CSP, but this time, we are missing upstream fixes... so I'll try to sync with upstream!

This patch only fixes the original issue being reported in #685's bugzilla ticket.