[Draft] Show replies in context of their threads

[gaearon]

This moves replies in feeds to work much more similarly to Twitter.

Not ready for review yet.

What it looks like

• Replies never show up in isolation — a reply is always seen in context of its root thread
• Slices are deduped by thread — with default settings, a friend reply essentially "bumps" a thread
• Threads never get torn apart, whether due to pagination or different author
• For long convos, you see root + last two items, which gives you better context than 1,2...N

TODO

• Proof of concept
• Address TODOs in code
• Handle blocks (for all levels), not found
• Handle Posts, Replies tabs
• Check feeds, lists
• Check self-threads
• Check how deduping interacts with self-reposts
• Double-check filtering still makes sense
• Simplify data structures
• Fix the "self-reply to a non-follow" problem
• Consider dropping reply settings

new_thread.mov

What it fixes

Torn replies like this will no longer happen:

Replies without context like this will no longer happen:

[render]

Your Render PR Server URL is https://social-app-pr-4835.onrender.com.

Follow its progress at https://dashboard.render.com/web/srv-cqhbgf6ehbks73950ge0.

[github-actions]

Old size	New size	Diff
7.01 MB	7.01 MB	-736 B (-0.01%)

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