Improve defaults and UX around local gateway redirect

[lidel]

There is an argument that installing IPFS Companion should not degrade browsing performance,
but augment it: recover when HTTP server is down, cache for later offline use.
Avoiding HTTP connection to the original server comes with trade-offs and surprises, which means it should be a user choice.

Food for thought

• make redirect to local gateway opt-in instead of opt-out when user has ipfs-companion installed
  (by default, only used as a recovery only when original server is down)
• indicate to user when page has IPFS resources or is a DNSLink website, and suggest "opening via IPFS"
  • allow enabling redirect to local gateway (per Origin or globally)
  • remember the choice, provide UI for changing it later

Rationale

• redirect to local gateway should be a conscious decision made by user, just like it is in Brave, and not a surprise, like it is in MV2 Companion
• safer default would avoid breaking websites which think they support IPFS, but are not (misconfigured or outdated DNSLink, JS code not working correctly with redirected resources, browser privacy extensions clashing with Companion logic.. list is long)

Prior art

• current behavior in MV2 is to provide opt-out toggle per Origin or globally:

  

• browser action indicator (loaded via IPFS / available on IPFS) – Add an indicator to show that resources have been loaded over IPFS #662

• Brave browser showing "IPFS" button on https://en.wikipedia-on-ipfs.org/ and other DNSLink pages. This is making the use of local gateway a conscious opt-in: