Inform users about v3 address migration in Onion Location
When a user loads a v2 onion address and the page send an onion-location containing a v3 address, then Tor Browser should show the user a message that encourages them to use the v3 address and (maybe?) update their bookmarks?
Ideally, the site would automatically (301) redirect users from the v2 -> v3 address, but using onion-location is a softer/friendlier path for migration.
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