Add merge trains empty state and polling

What does this MR do and why?

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

In this MR I add a empty state component that shows when there are no cars on a train. I also add the query polling to get the most up-to-date MRs/CI statuses on the train. Polling should only happen for the current active tab.

Progress

Part One: !149025 (merged)

Part Two: !151940 (merged)

Part Three: !152400 (merged)

Part Four: !156064 (merged)

Part Five: !156521 (merged)

Part Six: !157209 (merged)

Part Seven: You are here

Part Eight: 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

When there are no cars

When there are cars

When new branch is selected and there are no cars

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 a merge train and click "View merge details" in the widget on the MR.
3. Verify empty state shows when there are no cars
4. Verify branch name updates when switching branches
5. Verify polling only happens on the active tab in the network tab

Related to #454392 (closed)