Remove source branch or indicate removed source branch in merged and closed merge requests

• For both merged and closed merged requests, indicate that the source branch is removed or provide the ability to remove it.
• Update the UI to below. (Currently for merged merge requests, the functionality is already there, but update the UI to below).
• In particular, indicate the branch name in the widget itself.

Designs

Details:

• Branch notation, 130px max width, ellipses overflow, font-weight 600, monospaced
• Tooltip on hover button and branch name, stating full branch name