Add new merge trains route

What does this MR do and why?

First MR in an effort to introduce the ability to visualize merge trains for a project/target branch.

This MR creates the new route behind a feature flag and introduces the FE structure with a basic Vue app to be iterated on.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

With flag enabled and MT enabled

With flag disabled

How to set up and validate locally

1. Enable merge trains and the FF merge_trains_viz
2. Visit http://gdk.test:3000/root/ci-project/-/merge_trains
3. Ensure you can see view
4. Disable the flag and ensure you see a 404 status

Related to #454179 (closed)