Add Link header to list timelines

As both soapbox-fe and mastofe use Link response header to determine whether there are more list items, having it implemented for /api/v1/timelines/list/:id is required for actual compatibility with Mastodon API. Right now it only lets users of Mastodon API implementations see 20 latest posts from a list. Related to mastofe#89 and https://gitlab.com/soapbox-pub/soapbox-fe/-/issues/207