Update message on MR widget about merge train position

Summary

Recently, we've improved the message on MR widget about merge train position to clearly indicate that the position is dynamically changed by the index on the train.

This issue is about a further improvement.

Proposal

• In the MR widget, update the merge train message as defined below:

Current	New
In the merge train at position #{position}	Added to the merge train. There are #{position - 1} merge requests waiting to be merged

If the merge request is at position 1 (as in, it starts the merge train), the message should be displayed as:

Current	New
In the merge train at position 1	A new merge train has started and this merge request is the first of the queue.

Out of scope

• A link to the MR in front of it so you can traverse the train.
• An ETA in minutes for the merge.

Documentation

Yes, this will require updates in the documentation.

https://docs.gitlab.com/ee/ci/merge_request_pipelines/pipelines_for_merged_results/merge_trains/#how-to-view-a-merge-requests-current-position-on-the-merge-train