timeline backfilling/pagination support

[Treeki]

This is a first draft of support for backfilling and paginating timelines so that viewing tweets toots posts skeets in Ivory is a more comfortable experience.

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Requests to the home timeline with a min_id specified (and no max_id or since_id) are treated as "I want the latest posts", and it tries to fetch as many as possible and deliver them in one request, using the same logic that I wrote for BirdBridge (https://github.com/Treeki/BirdBridge/blob/main/main.ts#L129-L207).

I also generate a Link header to make Ivory happy - this gets rid of the issue where it's perpetually stuck in a loading state and won't allow you to manually refresh or show the amount of unread posts.

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What's still missing:

• Support for this same system on other timelines - it should work for custom timelines, for users' own timelines, and probably also for notifications
• The ability to go back in time: it serves a rel="next" link with a max_id field which in theory should work, but Skybridge won't use it yet

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Using the 'next' links is tricky because Bluesky requires cursors.

In theory you could ignore max_id and just supply the cursor as a non-standard parameter in the rel="next" link, but I haven't tested if this will work with Ivory.

I know that rel="prev" links definitely need to follow the standard format for Ivory to work correctly (as it will try to generate its own in certain cases, based off its newest known post ID) but I don't think they do the same for rel="prev". It's probably worth a try at least!

Tried this and it works, so that technique seems viable - I can now scroll further down in the timeline!

[felixmueller]

@videah @Treeki ＋1️⃣ for merging this PR if it's working! Finally a fixed loading indicator and refresh function in Ivory 👌