Add merge trains viz link to the pipeline header

What does this MR do and why?

Add link to the merge trains viz on the pipeline header when the pipeline is a merge train pipeline.

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

Merge train pipeline

Non merge train pipeline

How to set up and validate locally

1. Visit a pipeline that is a merge train pipeline (verify link exists)
2. Visit a pipeline that is a non merge train pipeline (verify link does not exist)

Related to #469149 (closed)