"id" is used for the canonical link to the AS2 representation of an object.
"url" is typically used for the canonical link to the HTTP representation.
It is what we use, for example, when following the "external source" link
in the frontend. However, it's not the link we include in the post contents
for quote posts.
Using URL instead means we include a more user-friendly URL for Mastodon,
and a working (in the browser) URL for Threads
