New user home feed fixes (#2421)

* Readd discover to default feeds for new users

* Fallback just to discover in home when mergefeed is disabled

* Always provide a cursor with mergefeed (no 'end of feed' condition)

src/lib/api/feed/merge.ts

@@ -98,7 +98,7 @@ export class MergeFeedAPI implements FeedAPI {
     }
 
     return {
-      cursor: posts.length ? String(this.itemCursor) : undefined,
+      cursor: String(this.itemCursor),
       feed: posts,
     }
   }

src/lib/constants.ts

@@ -117,8 +117,8 @@ export async function DEFAULT_FEEDS(
   } else {
     // production
     return {
-      pinned: [],
-      saved: [],
+      pinned: [PROD_DEFAULT_FEED('whats-hot')],
+      saved: [PROD_DEFAULT_FEED('whats-hot')],
     }
   }
 }