Fix extending user-list feeds.

Fix extending user-list feeds.

In lists of followers, favers, boosters etc, the ids you have to pass
in the 'max_id' or 'since_id' URL query parameters are not the same as
the account ids you get back. That's not surprising, now I think about
it, since the chronological order in which users did things has
nothing to do with any intrinsic ordering on the users themselves.

For status feeds, they seemed to match, so I got away without having
to pay attention to the Link: headers in the HTTP responses. But now I
realise I do have to worry about that after all. Happily, there's a
handy Rust crate that saves me having to do the work of parsing that
complicated header type!