Replace ipfs.io with dweb.link

[lidel]

I believe we are at a stage where we should always use dweb.link on the web.

Due to historical reasons we have separate settings for "default public gateway" and "default public subdomain gateway". Those should be merged and dweb.link should be the default gateway for use in browsers.

This means:

• we would replace ipfs.io with dweb.link as the default public gateway
• remove subdomain one from Preferences entirely
• ensure we always copy content-path URL http://dweb.link/ipfs/* instead of subdomain version (to leverage future-proof CID fixups in go-ipfs)
• 👉 ensure DNSLink names work on public gateways – 🛑 NEEDS: TLS for DNSLink websites loaded via public subdomain gateways in-web-browsers#169

Ref https://docs.ipfs.io/how-to/address-ipfs-on-web/