Code Insights: Add backend insight story card
Created by: vovakulikov
Closes part of https://github.com/sourcegraph/sourcegraph/issues/23189
Context
This PR does preparation work for the drill-down filters panel at the backend insight card
- Change the gql method API in order to be able to mock the fetch method in storybook stories
- Adds the backend card storybook story
- Fix global styles usage (now all global styles must be included via one single insight styles file at the top level of /insights dir)