Display one file at a time when reviewing code

From this discussion

Problem

When reviewing large merge request that encompass a large number of files, the MR/diff section becomes cluttered and it's hard to keep track of which files need attention. Also performance of the page is slower due to the number of changes/files that need to be displayed.

Proposal

Add a user settings which allows users to view a single file at a time on the "Changes" tab in merge request, to be user-specific (user >> settings >> preferences >> behavior)