WIP Code Insights: Migrate code insights pages to new GQL based backend

Created by: vovakulikov

Context

This is integration PR that suppose to be the main entry point for any setting cascade/GQL migration ticket/issue. After that PR by default, all users will be using the new GQL backend for code insights. But in case if some customers will tell us that something has been lost or for some reason, our API doesn't work with data properly we can roll back that API by a feature flag experimentalFeatures.codeInsightsGqlApi. But we expect that we won't be needed to do that but also we still keep our old API at least for the next release.

Next steps (child PRs or changes in this branch)

• Move away from stating cascade on the dashboard page
  • Dashboard page itself
  • Dashboard creation UI
  • Dashboard edit page
• Code Insights creation UI https://github.com/sourcegraph/sourcegraph/pull/26422
  • Search insight creation form
  • Lang stats creation form