Improve sidebar for comments index to not rely on `indexSidebarLimit`

[Josh Crawford]

There’s a different query when generating the sidebar in the comments element index, which limits how many comments to look at, to get their owner element. This query is very computational expensive, so we’ve added a cap to it indexSidebarLimit, but that's not a great solution, as it only relies on the last 25 comments to generate these groups.

Instead, come up with a better way (direct querying) to output the sidebar groups.