Fix AP outbox pagination

This fixes the ActivityPub /outbox endpoint pagination.

Since outboxes are sorted chronologically descending (newest first), the max id will be found at the top of the list, instead of at the bottom, so the next page link should have the max_id of whatever the last (in the list) post on the previous page was.

Note that there is also an issue with the id field which I haven't attempted to tackle: requests with max_id do not include the max_id activity, so if you follow the id link you'll actually go to a different page.