Fix position of loading indicator for issue notes

What does this MR do and why?

When paginated_issue_discussions is enabled, we should show the loading indicator at the top if the notes are ordered by newest first.

Ideally, we'd be fetching the newest notes first, but the backend does not support that right now and note ordering is purely frontend. So I just fixed the loading indicator for now.

Screenshots or screen recordings

How to set up and validate locally

1. Enable the paginated_issue_discussions feature flag.
2. Open an issue with more than 20 discussions.
3. Select the "Newest first" sort order.

MR acceptance checklist

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