Timeline: always insert new status IDs at the end

Summary

In both Mastodon and Pleroma, status IDs are time-based, meaning you can alphabetically sort the ID to get the statuses in chronological order. Ditto, so far, does not have this quality. So to the greatest extent possible we should not rely on it in Soapbox for compatibility with Ditto, and potentially other backends.

This makes timelines not infer the insert position based on IDs.