Recover from dead Subdomain Gateways

[lidel]

Continuation of #640, and #797, cc https://github.com/ipfs/ipfs/issues/337, ipfs/in-web-browsers#89

Motivation

We now have a framework for recovering from failed HTTP responses to Path Gateways and DNSLink websites. To make it feature-complete we should do the same for Subdomain Gateways.

Details

https://bafybeiemxf5abjwjbikoz4mc3a3dla6ual3jsgpdr4cjr3oz3evfyavhwq.ipfs.broken-example.com/wiki/

Above URL fails at DNS lookup stage because broken-example.com domain name does not exist. This produces NS_ERROR_UNKNOWN_HOST on Firefox and net::ERR_NAME_NOT_RESOLVED on Chromium, which are already supported since #797.

We want to recover from this by redirecting failed subdomain-based gateways to dweb.link (our canonical subdomain gateway):
https://{cid}.ipfs.dweb.link/path/

In the future, when we have support for {cid}.ipfs.localhost (ipfs/in-web-browsers#109), we will also redirect to local node, but for now redirect to dweb.link should be enough.

cc @colinfruit