Consider tightening the CSP policy when serving untrusted content

[jvoisin]

Currently, when serving untrusted content, miniflux uses the following CSP policy:

$ git grep '"Content-Security-Policy", `default-src'
internal/http/response/html/html.go:40: builder.WithHeader("Content-Security-Policy", `default-src 'self'`)
internal/http/response/html/html.go:64: builder.WithHeader("Content-Security-Policy", `default-src 'self'`)
internal/ui/feed_icon.go:29:            b.WithHeader("Content-Security-Policy", `default-src 'self'`)
internal/ui/proxy.go:147:               b.WithHeader("Content-Security-Policy", `default-src 'self'`)
$

It would be nice to use sandbox instead.