Show Security warning when rebasing from forked MR

What does this MR do and why?

This MR shows a security warning modal when rebasing an MR from a forked project.

Screenshots or screen recordings

How to set up and validate locally

1. Create project
2. Fork project from another user
3. Open MR that creates a CI file etc.
4. Switch back to user that created project
5. Click rebase button on the MR from the forked repo

More specific use-case setup instructions can be found in issue #359955 (closed)

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 #359955 (closed)