Add merge trains visualization entry point

What does this MR do and why?

We are working on building a new feature, merge train visualization.

In this MR I add the ability to reach the view after adding an MR to a merge train.

Progress

Part One: !149025 (merged)

Part Two: !151940 (merged)

Part Three: You are here

Part Four: TBD

Part Five: TBD

Part Six: TBD

See the entire epic here where I have mapped out this feature build.

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

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

New message when multiple MRs are on the train

How to set up and validate locally

1. Enable merge trains and the FF merge_trains_viz
2. Setup merge trains and add an MR to the train
3. See new link and message. Ensure link goes to the new view
4. Disable the flag and ensure you see old text/no link

Related to #460319 (closed)