Fix PostStream Reply Scroll (#2366)

- Add an index to reply placeholder so we can scroll to it directly when replying.
- Stop pretending that the currently broken `bottom` scroll functionality works, and explicitly call it `reply` scrolling to be clearer
- Directly get target from state
- Explicitly scroll to placeholder on reply
- Clean up scrollToItem code a bit
- Account for edge case where index is undefined when scrolling to post

Co-authored-by: Wadim Kalmykov <36057469+w-4@users.noreply.github.com>

js/src/forum/states/PostStreamState.js

@@ -96,7 +96,9 @@ class PostStreamState {
     // If we want to go to the reply preview, then we will go to the end of the
     // discussion and then scroll to the very bottom of the page.
     if (number === 'reply') {
-      return this.goToLast();
+      const resultPromise = this.goToLast();
+      this.targetPost.reply = true;
+      return resultPromise;
     }
 
     this.paused = true;