insights: use experimentalFeatures flags rather than Feature flags for aggregations

Created by: Joelkw

"Feature Flags" in the admin portal are for A/B testing and we don't get that info via pings, and also are not as easy to set as experimental settings flags. (See also When to use Feature Flags.)

We should use experimental settings for both:

1. Whether we show aggregations at all.
2. Whether we allow it to run proactive aggregations

Both should be able to be set at the admin or user level.

/cc @joelkw @felixfbecker @vovakulikov