[Design] Merge details layout enhancements

Proposal

Highlight key information from the merge details that is currently in the merge check widget.

Goal

1. When landing the merge request page, users should know what the "state" of the merge request is without scrolling
   • Currently users will need to scroll and find the merge widget on the page to read the status

Mock up

Current	Proposal

There are other states not illustrated here such as merged and closed. They will come once we refine the layout.

Questions to answer

1. @francoisrose Do you imagine the Work Items line would also be shown if there aren't any linked issues? For me, it's often just as valuable to know at a glance that there isn't anything linked.