URI handler fallback

[SoniEx2]

trafficstars

Webpages with a <meta protocol="PROTOCOL" default="DEFAULT?url=%s" /> tag should have all their PROTOCOL:[...] links point to DEFAULT?url=PROTOCOL:[...], unless the user has added a protocol handler for the given protocol.

I mainly need this for web+ganarchy and web+mastodon, but there's no reason to restrict them to only these 2.

This improves the UX around URI handlers and makes them actually usable in the wild. See also: https://github.com/whatwg/html/issues/4584