Add keybinding for next/previous file in merge request diff

@jkleckner suggests https://gitlab.com/gitlab-org/gitlab-ce/issues/40844#note_66495109

And the Gerrit keybindings for "]" and "[" to forward/back through the files would be nice

I use keyboard shortcuts to jump between changes in my local editor and diff tools. This would make doing the same in GitLab possible!

Proposal

When viewing a merge request and the view the changes

• ] or j moves to the next changed file
• [ or k moves to the previous changed file

Links / references

Highlight currently focused/viewed file in file tree: https://gitlab.com/gitlab-org/gitlab-ce/issues/52485