Add feature flag to disable proactive aggregation insights

Created by: unclejustin

Closes https://github.com/sourcegraph/sourcegraph/issues/40427 Closes https://github.com/sourcegraph/sourcegraph/issues/40683

Test plan

• Unit test should pass
• Feature flag search-aggregation-filters need to be enabled
  • Go to site-admin/feature-flags create a boolean feature flag called search-aggregation-filters set to true
• Search for anything. i.e. "hello" Confirm aggregation card shows in the side panel
• Enable disable-proactive-insight-aggregation
  • Go to site-admin/feature-flags create a boolean feature flag called disable-proactive-insight-aggregation set to true
• Run any search
• Confirm only buttons show in the side bar and explanatory text show

• Select a grouping mode by clicking one of the buttons
• Confirm chart displays

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.