Code Insights: clean up code insight creation UI and add high-level compute page

Created by: vovakulikov

Closes: https://github.com/sourcegraph/sourcegraph/issues/38049 Preparation for: https://github.com/sourcegraph/sourcegraph/issues/37965

This PR slightly changes our import/export system around the creation UI pages in a way to simplify imports in the creation UI components. It also adds a high-level page component for the compute-powered insight.

Test plan

• Make sure that you can see compute insight creation UI page skeleton on /insights/create/compute

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.