Fix the merge button dropdown

What does this MR do?

Fixes the bug detailed in https://gitlab.com/gitlab-org/gitlab-ce/issues/64970. This merge request makes two distinct changes:

1. Fixes the merge button dropdown trigger so that it no longer is rendered if the dropdown itself is not rendered.
2. Allows users to select "Merge Immediately" when using merge trains after the MR's pipeline has successfully completed.

This MR has a CE backport

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/31248

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64970.