Add route, controller action, and views for MR pipelines

app/assets/javascripts/merge_request_tabs.js.coffee

app/assets/javascripts/merge_request_widget.js.coffee

app/controllers/projects/merge_requests_controller.rb

app/views/projects/commit/_pipelines_list.haml

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

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

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

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

config/routes.rb