Add empty import details page behind a feature flag

What does this MR do and why?

• Add import_details_page feature flag.
• Add route, controller action and boilerplate frontend code.

Screenshots or screen recordings

How to set up and validate locally

• Go to http://127.0.0.1:3000/import/github/details. The page should render 404 Not Found.
• Enable the feature flag and refresh the page:

Feature.enable :import_details_page

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #386748 (closed)