Show system notes for Merge Trains

Problem to solve

In order to keep track of actions taken on a Merge Train in the merge request widget, system notes should be added to the Discussion tab of a merge request. The notes should inform when a merge train is started / a merge request is added to a merge train.

This interaction is related to https://gitlab.com/gitlab-org/gitlab-ee/issues/9186

System notes

Scenario	Message
If no previous existing merge train exists	*John Doe* @jdoe started a merge train
If a merge train already exists, and a merge train pipeline succeeds	*John Doe* @jdoe added this merge request to the merge train at index #{merge_train_index}
When a user removes a merge request from a merge train	*John Doe* @jdoe removed this merge request from the merge train
When the pipeline of a merge request on the merge train fails	This merge request has been removed from the merge train because #{failure_reason}`