Expose in UI branch-name to easy copy during scrolling long Merge Request View

Problem to solve

During reviewing merge request when I am in the middle of MR and I want to checkout locally code from this branch, I have to scroll to the begining of the MR. This is very inconvenient.

Intended users

Developers

Further details

It will be nice to make branch-name availalbe to copy from place (on UI) which is visible from every place of Merge Request View. E.g. on the right panel under "Reference" label could be added smth. like "Branch name" and button to copy it into clipboard.

Permissions and Security

No change in permissions needed.