Use viewed state from file header in MRs file browser

Context

Before we had the ability to mark diff files as viewed (https://docs.gitlab.com/ee/user/project/merge_requests/reviewing_and_managing_merge_requests.html#mark-files-as-viewed) we would make the file names in the file browser bold/unbold based on if the user has viewed them or not.

Problem

Today there's a disconnect between the “Mark as viewed” checkbox and the files in the file browser. They don't communicate and so the bolded file names in the file browser are not understandable. Usually the bold/regular toggle is associated with things that are viewed/not viewed or read/unread.

Proposal

Bold file names if their “Viewed” checkbox is unchecked and unbold them if the checkbox is checked.