The gateway accepts an X-Ipfs-Path-Prefix header,
and assumes that it is mounted in a reverse proxy
like nginx, at this path. Links in directory listings,
as well as trailing-slash redirects need to be rewritten
with that prefix in mind.

We don't want a potential attacker to be able to
pass in arbitrary path prefixes, which would end up
in redirects and directory listings, which is why
every prefix has to be explicitly allowed in the config.

Previously, we'd accept *any* X-Ipfs-Path-Prefix header.

Example:

We mount blog.ipfs.io (a dnslink page) at ipfs.io/blog.

nginx_ipfs.conf:

    location /blog/ {
        rewrite "^/blog(/.*)$" $1 break;
        proxy_set_header Host blog.ipfs.io;
        proxy_set_header X-Ipfs-Gateway-Prefix /blog;
        proxy_pass http://127.0.0.1:8080;
    }

.ipfs/config:

    "Gateway": {
        "PathPrefixes": ["/blog"],
        // ...
    },

dnslink:

    > dig TXT _dnslink.blog.ipfs.io
    dnslink=/ipfs/QmWcBjXPAEdhXDATV4ghUpkAonNBbiyFx1VmmHcQe9HEGd

License: MIT
Signed-off-by: Lars Gierth <larsg@systemli.org>