Code Insights: Add header actions to the insight standalone page

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/34759 Based on https://github.com/sourcegraph/sourcegraph/pull/34783

This PR simply adds a header component to the page. The header layout looks slightly different from the original designs in order to simplify implementation and will be improved after the insight standalone page v1 (I think we will handle this right after the 3.40 cc @AlicjaSuska @Joelkw)

Test plan

• Make sure that all actions (copy link, edit, delete insight) over insight work properly
• Make sure that the header layout looks on all viewports

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.