Code Insights: Implement delete insights mutation handler in gql api class
Created by: vovakulikov
Closes part of https://github.com/sourcegraph/sourcegraph/issues/27415 (known issue #3 - After search insight creation the dashboard page fails with runtime error about the premature observable cancelation)
Closes https://github.com/sourcegraph/sourcegraph/issues/27217
Background
In this PR we implement a delete insight mutation handler in our GQL API class. We had to slightly change the interface of getInsights
method in order to avoid problems with re-fetching and Apolo cache.
For reviewers: please test deleting of insight with new API and with old setting-based API to avoid regressions around insights fetching on the dashboard page since we've changed some methods of setting-based API as well to implement the latest common API interface.