Code Insights: Add insight delete confirmation step

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/21515

Context

At the moment in main if you click delete insight we immediately delete your insight without any confirmation step. As @felixfbecker noticed

there is no "undo" and it's easy to miss the "edit" button and accidentally delete

So this PR adds a simple confirmation step via window.confirm method.