Allow commenting on older versions of the diff and comparisons between diff versions

app/assets/stylesheets/pages/diff.scss

app/assets/stylesheets/pages/merge_requests.scss

app/controllers/projects/merge_requests_controller.rb

app/helpers/notes_helper.rb

app/models/concerns/discussion_on_diff.rb

app/models/concerns/note_on_diff.rb

app/models/diff_discussion.rb

app/models/diff_note.rb

app/models/legacy_diff_discussion.rb

app/models/merge_request.rb

app/models/note.rb

app/views/discussions/_diff_with_notes.html.haml

app/views/discussions/_discussion.html.haml

app/views/projects/diffs/_line.html.haml

app/views/projects/merge_requests/show/_versions.html.haml

changelogs/unreleased/dm-comment-on-diff-versions.yml

spec/features/merge_requests/versions_spec.rb

spec/helpers/notes_helper_spec.rb

spec/models/diff_discussion_spec.rb

spec/models/diff_note_spec.rb