Code Insights: "Remove from dashboard" feature

Created by: vovakulikov

Background

Currently, there is only one way to remove/unassing insight from the dashboard. You have to do that via the dashboard context menu.

It seems like we can easily add the Remove from dashboard action item to the insight context menu.

Technical details

We should reuse jsonc setting cascade files logic from the dashboard add or remove insight modal. https://sourcegraph.com/github.com/sourcegraph/sourcegraph/-/blob/client/web/src/insights/core/settings-action/dashboards.ts?subtree=true