Code Insights: Extract dashboard logic to the setting based backend api

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/25874

Context

This PR changes our API in a way to be able to change code insights API in the feature completely. For example at the moment almost all that is related to the code insights and storing insights and dashboard it's built on top of the settings cascade object. In the future, we want to introduce a new API - GQL based. But we still want to support setting cascade-based API for some cases when users don't like it or they lose some data that they already have with our new GQL API (So it's a fallback plan). To be able to do that we need to change our approach to the API (sync -> async migration)

New approach

The first thing that we should do probably to support that it's to change our approach from sync model (when we have all data in all components always - setting cascade model) to the system where all operations and all selectors to the setting cascade are async. We need to do that to be able to change our avoid further change when wo go to GQL API (where all operations are async)