Filter merge trains by branch param

What does this MR do and why?

Filter merge trains by branch param

Adds ability to save URLs and filter merge trains by branch query param.

Changelog: added

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. Have multiple target branches for project.
2. Visit http://gdk.test:3000/<namespace>/<project>/-/merge_trains?branch=<branch-name>&tab=active and ensure branch selector value is set and query is called correctly.
3. Use branch selector and ensure query param branch is updated and any other query params are retained.

Related to #493393 (closed)