Factor our feed source model (#1887)

* Refactor first onboarding step

* Replace old FeedSourceCard

* Clean up CustomFeedEmbed

* Remove discover feeds model

* Refactor ProfileFeed screen

* Remove useCustomFeed

* Delete some unused models

* Rip out more prefs

* Factor out treeView from thread comp

* Improve last commit

src/lib/hooks/useCustomFeed.ts

@@ -1,18 +0,0 @@
-import {useEffect, useState} from 'react'
-import {useStores} from 'state/index'
-import {FeedSourceModel} from 'state/models/content/feed-source'
-
-export function useCustomFeed(uri: string): FeedSourceModel | undefined {
-  const store = useStores()
-  const [item, setItem] = useState<FeedSourceModel | undefined>()
-  useEffect(() => {
-    async function buildFeedItem() {
-      const model = new FeedSourceModel(store, uri)
-      await model.setup()
-      setItem(model)
-    }
-    buildFeedItem()
-  }, [store, uri])
-
-  return item
-}