broken subresource redirects when CSP rule does not safelist localhost

[lidel]

See https://github.com/SmaugPool/pool.pm/issues/20#issuecomment-1644792534 for repro steps.

CSP blocks image request and it does not load.

Potential fix is to ignore subresource requests with content-security-policy header that blocks loading resource from user's gateway. That way it loads from original URL and we don't break websites.

[SgtPooki]

@whizzzkid you should probably peek at this one while working on MV3