Add option to start a new discussion on an MR

app/assets/javascripts/files_comment_button.js

app/assets/javascripts/notes.js

app/controllers/projects/commit_controller.rb

app/controllers/projects/discussions_controller.rb

app/controllers/projects/issues_controller.rb

app/controllers/projects/merge_requests_controller.rb

app/controllers/projects/notes_controller.rb

app/controllers/projects/snippets_controller.rb

app/finders/notes_finder.rb

app/helpers/notes_helper.rb

app/models/commit.rb

app/models/commit_discussion.rb

app/models/concerns/note_on_diff.rb

app/models/concerns/noteable.rb

app/models/concerns/resolvable_note.rb

app/models/diff_discussion.rb

app/models/diff_note.rb

app/models/discussion.rb

app/models/discussion_note.rb

app/models/individual_note_discussion.rb