Implement new sidebar for merge request page

app/assets/stylesheets/framework/common.scss

app/assets/stylesheets/framework/selects.scss

app/assets/stylesheets/pages/merge_requests.scss

app/controllers/projects/issues_controller.rb

app/controllers/projects/merge_requests_controller.rb

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

app/views/projects/merge_requests/_show.html.haml

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

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

app/views/projects/merge_requests/update.js.haml