Unixfs symlinks on Gateway

[ghost]

Right now the gateway doesn't handle symlinks, it just passes everything that's not a directory to unixfs/io.DagReader, which refuses to work on symlinks.

internalWebError: cannot currently read symlinks

Needed for the cdn.media.ccc.de mirror.

[lidel]

Some relevant discussion happened in #3508

Quick takeaways on top of that thread:

• I believe a HTTP redirect is an okay solution, however "the root" that stops .. from going further should end at the root of each CID, not the namespace
  • So in /ipfs/CID/foo/bar the /ipfs/CID/ is the content root, not /ipfs/ (CIDs should not share root for security reasons)
• modern solution should account for native/subdomain and path addressing (https://docs.ipfs.io/how-to/address-ipfs-on-web/)