Fastly: Include security headers for dl

[ameukam]

Add Security headers to the VCL service.

Ref:

• https://web.dev/secure/
• https://infosec.mozilla.org/guidelines/web_security

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[ameukam]

cc @upodroid @BenTheElder

[BenTheElder]

if we're not doing auth or hosting web authentication this is kind of /shrug?

what's the case where we care if a web browser tries to do cross-origin here?

[ameukam]

Yes but IMHO it doesn't hurt to have them. We don't know if in the future for examples static pages we host will be become dynamic content.