The previous assertion that:
> if it's a complicated thing and the `activity.id` doesn't match, I
> think we're fine rejecting the activity
was wrong: at least peertube sends activities that have `url` as an
array of objects.
Notice that this does *not*, in fact, fix#512: the peertube activity
does not contain its short URL (`https://example.com/w/someid`), so
there's no way to confirm that it is the activity we requested.
