[APP-705] Metrics revamp pt2 (#896)

* export track function from analytics.tsx

* fix create account tracking

* fix tracking sign in

* add custom feed events

* fix type errors

* refactor create post event

* add profile follow & unfollow events

* refactor PostsFeedSliceModel into its own file

* refactor PostThreadItemModel into its own file

* reorganize code a lil bit

* refactor post-thread-item to use post-feed-item model under the hood

* add post events

* add post reply tracking

* track custom feed load more

* track list subscribe and unsubscribe

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

@@ -9,10 +9,8 @@ import {
 } from 'react-native'
 import {AppBskyFeedDefs} from '@atproto/api'
 import {CenteredView, FlatList} from '../util/Views'
-import {
-  PostThreadModel,
-  PostThreadItemModel,
-} from 'state/models/content/post-thread'
+import {PostThreadModel} from 'state/models/content/post-thread'
+import {PostThreadItemModel} from 'state/models/content/post-thread-item'
 import {
   FontAwesomeIcon,
   FontAwesomeIconStyle,