Add Discussion model to represent MR/diff discussion

app/assets/javascripts/notes.js.coffee

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/helpers/diff_helper.rb

app/helpers/notes_helper.rb

app/models/concerns/note_on_diff.rb

app/models/discussion.rb

app/models/note.rb

app/views/discussions/_diff_discussion.html.haml

app/views/discussions/_diff_with_notes.html.haml

app/views/discussions/_discussion.html.haml

app/views/discussions/_notes.html.haml

app/views/discussions/_parallel_diff_discussion.html.haml

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

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

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

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

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