Threads v2 cleanup (#8902)
* Delete root PostThread component
* Remove PostThreadItem, migrate DebugMod to use new components
* Remove other unused components
* Move PostThreadFollowBtn to new home
* Move PostThreadComposePrompt to new home
* Remove gate
* Keep naming in DebugMod
* rm v1 prefs
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Co-authored-by: Samuel Newman <mozzius@protonmail.com>
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