CE backport for gitlab-org/gitlab-ee!12156: Update the merge request widget's "Merge" button to support merge trains

Nathan Friendrequested to merge
9186-implement-atmtwps-state-to-mr-widget into master

What does this MR do?

CE backport for https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/12156

Edited by Nathan Friend

Merge request reports