Build active train and merged tabs with basic table

What does this MR do and why?

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

In this MR I add tabs and inject a very basic table. This will be built upon in future MRs when we have a working BE. In the spirit on iteration/maintainable MRs, this feature will be built in multiple MRs.

Progress

Part One: !149025 (merged)

Part Two: You are here

Part Three: TBD

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

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
3. Ensure you can see the view, with tabs and table
4. Disable the flag and ensure you see a 404 status

Related to #459884 (closed)