R1:ab579f251110

Never generate file download forms which point to the CDN domain, tighten "form-action" CSP

Summary:
Depends on D19155. Ref T13094. Ref T4340.

We can't currently implement a strict `form-action 'self'` content security policy because some file downloads rely on a `<form />` which sometimes POSTs to the CDN domain.

Broadly, stop generating these forms. We just redirect instead, and show an interstitial confirm dialog if no CDN domain is configured. This makes the UX for installs with no CDN domain a…