chore(grouping): Use optimized grouping logic for 20% of non-transitioning projects

[lobsterkatie]

This enables using the new grouping logic (_save_aggregate_new) for 20% of projects not currently in a grouping config transition. (It's already rolled out to all transitioning projects.) Unlike for transitioning projects, _save_aggregate_new doesn't actually do anything different than _save_aggregate does with events from non-transitioning projects, so this change is essentially just switching the projects in question to a refactored version of what's already happening.