Allow filtering the pipeline list by `merge train` tag

Problem

As a Developer working with Merge Trains, I want to see where my MR's pipeline is on the train, so I can guesstimate how much time is left before it completes and can be tested/deployed/reviewed/etc.

Proposal

When viewing the list of pipelines, it would be nice to be able to filter them to only show the list of pipelines on the merge train.