insights: enable "group by" insights type

Created by: chwarwick

To enable the feature flow of saving a search aggregation as a "group by" insight. The feature flag or setting that disables the group by insight type needs to be removed or enabled.

If the setting remains and therefore could be disabled. The "Save Insight" in the search aggregation UI must also honor this.

/cc @joelkw @felixfbecker @vovakulikov @unclejustin