Extract LegacyDiffNote out of Note

app/assets/javascripts/notes.js.coffee

app/assets/stylesheets/pages/notes.scss

app/controllers/projects/commit_controller.rb

app/controllers/projects/compare_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/controllers/projects/notes_controller.rb

app/finders/notes_finder.rb

app/helpers/diff_helper.rb

app/helpers/notes_helper.rb

app/models/legacy_diff_note.rb

app/models/note.rb

app/views/notify/note_merge_request_email.html.haml

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

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

app/views/projects/notes/_commit_discussion.html.haml

app/views/projects/notes/_diff_notes_with_reply.html.haml

.../projects/notes/_diff_notes_with_reply_parallel.html.haml

app/views/projects/notes/_discussion.html.haml

app/views/projects/notes/_form.html.haml

app/views/projects/notes/_note.html.haml