Improve defaults and UX around public gateway redirect

[lidel]

This is similar to #1072 but about what happens when user has no local node running.

Current behavior

When user has ipfs-companion enabled but not local IPFS node running, it will redirect every /ipfs/{cid} request to public gateway.

This means we redirect every user to our gateway because it is unlikely many people change the default.

Proposed change

• Make this opt-in, similar to Improve defaults and UX around local gateway redirect #1072, and use our gateway only if original one fails (i believe HTTP failure recovery is already implemented, just need to make redirect opt-in).
• (TBD) display gateway choice the first time user makes the decision?
  • test which gateways are working in user's jurisdiction/isp - similar to https://ipfs.github.io/public-gateway-checker/
    • this solves the problem of dweb.link or ipfs.io being blocked in some places