Make scroll handling contextual (#2200)

* Add an intermediate List component

* Fix type

* Add onScrolledDownChange

* Port pager to use onScrolledDownChange

* Fix on mobile

* Don't pass down onScroll (replacement TBD)

* Remove resetMainScroll

* Replace onMainScroll with MainScrollProvider

* Hook ScrollProvider to pager

* Fix the remaining special case

* Optimize a bit

* Enforce that onScroll cannot be passed

* Keep value updated even if no handler

* Also memo it

src/view/com/post-thread/PostRepostedBy.tsx

@@ -1,7 +1,8 @@
 import React, {useMemo, useCallback, useState} from 'react'
 import {ActivityIndicator, RefreshControl, StyleSheet, View} from 'react-native'
 import {AppBskyActorDefs as ActorDefs} from '@atproto/api'
-import {CenteredView, FlatList} from '../util/Views'
+import {CenteredView} from '../util/Views'
+import {List} from '../util/List'
 import {ProfileCardWithFollowBtn} from '../profile/ProfileCard'
 import {ErrorMessage} from '../util/error/ErrorMessage'
 import {usePalette} from 'lib/hooks/usePalette'
@@ -85,7 +86,7 @@ export function PostRepostedBy({uri}: {uri: string}) {
   // loaded
   // =
   return (
-    <FlatList
+    <List
       data={repostedBy}
       keyExtractor={item => item.did}
       refreshControl={