Update the merge request widget's "Merge" button to support merge trains

What does this MR do?

Updates the Merge button in the merge request widget to support merge trains:

This feature is effectively hidden behind the merge_trains feature flag.

This is one step towards the merge trains feature described in https://gitlab.com/gitlab-org/gitlab-ee/issues/9186.

UX specs: https://balsamiq.cloud/saotw6e/pjtd1bw/rED67

CE backport: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/27594