Improve UX when localhost gateway is offline

[lidel]

Problem

Right now, when one tries to open localhost URL and the gateway is down, they get a connection error.

Improvement proposal

IPFS Companion could catch when the local gateway is down, and ask the user if they would like to open the requested path using a public gateway, or in the case of DNSLinks, the upstream http server.

The intefrace should be similar to uBlock's interstitial, asking user if they want to allow tracking link Temporary, or Permanently. So if user wants to always recover using public gateway, and they select Permanently, they will be bothered by this interstitial only once.

Note: the same interstitial UI can be used for asking the user if they would like to load DNSLink website from original URL, or from local gateway.