Add merge trains viz feedback banner

What does this MR do and why?

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

In this MR I add a feedback banner and make a small tweak to the completed merge trains query.

Progress

Part One: !149025 (merged)

Part Two: !151940 (merged)

Part Three: !152400 (merged)

Part Four: !156064 (merged)

Part Five: You are here

Part Six: TBD

Part Seven: 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

How to set up and validate locally

1. Enable merge trains and the FF merge_trains_viz
2. Visit http://gdk.test:3000/namespace/project/-/merge_trains or add an MR to the train and click "View merge details" in the widget on an MR.
3. Ensure you can see the view, with tabs, table and feedback banner

Related to #464777 (closed)