Use Linked file for diff comments on the Overview tab

We should use a Linked file feature on the Overview tab for links to diff files. Example comment:

The .gitlab/ci/rails.gitlab-ci.yml file link should point to a Linked file (/diffs?file=<hash>#<line>), right now it points just to a line hash (?diffs#<line>).

This will also fully resolve Collapsed lines do not expand when linked (#480608 - closed), which is fixed only for the Linked file.