Refactor the merge section of the merge request widget

Problem

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20189 introduced a new design for the werge request widget info and pipeline sections

https://gitlab.com/gitlab-org/gitlab-ee/issues/5927 introduces a new design for the middle (report) part of the mr widget. Included in the mockups is an indication of how the bottom part (in order to get the mr merged) is going to look like.

This issue should tackle that design of the merge section of the mr widget in the same way as https://gitlab.com/gitlab-org/gitlab-ce/issues/46795 / https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20189

This redesign is preferably needed in order to support https://gitlab.com/gitlab-org/gitlab-ee/issues/3709