Fork suggestion becomes a modal

What does this MR do and why?

• uses custom GlModal instead of createAlert to specify post method on the fork button

References

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• Fork suggestion becomes page-level alert
• Prepare util functions for a fork suggestion alert

MR acceptance checklist

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Screenshots or screen recordings

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Before	After
fork_suggestion_before	Screen_Recording_2025-03-04_at_18.36.01

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. In rails console enable the experiment fully

   Feature.enable(:blob_overflow_menu)

2. Impersonate another user and choose a project where that user is a guest

3. Go to Project / Repository and open any file

4. Open the actions overflow menu and make sure Upload file and Delete file do not trigger their respective modals.

5. Make sure the fork suggestion is rendered, dismissible and forks the project on the primary action.

6. To compare with previous implementation disable blob_overflow_menu feature flag. Upload and Download controls will be located on the file viewer bar.

Related to #519316 (closed)