Paste a copied MR source branch name as code when pasted into a GFM form

When clicking the copy button to the right of the source branch name in the merge request widget, and subsequently pasting into a GFM form, it will paste as `source-branch-name` with backticks, which is later rendered as source-branch-name. When pasting anywhere that is not a GFM form, it will paste as source-branch-name without backticks as expected.

Similar to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9876.