cmd/go/internal/web: reject insecure redirects from secure origins

We rely on SSL certificates to verify the identity of origin servers.
If an HTTPS server redirects through a plain-HTTP URL, that hop can be
compromised. We should allow it only if the user set the -insecure
flag explicitly.

Fixes #29591

Change-Id: I00639541cca2ca034c01c464385a43b3aa8ee84f
Reviewed-on: https://go-review.googlesource.com/c/go/+/156838


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>