Added old dashboard toggle to new merge request dashboard

...ts/merge_request_dashboard/components/action_dropdown.vue

...ts/javascripts/merge_request_dashboard/components/app.vue

app/assets/javascripts/merge_request_dashboard/index.js

...ssets/javascripts/pages/dashboard/merge_requests/index.js

app/helpers/merge_requests_helper.rb

app/views/dashboard/merge_requests.html.haml

config/events/toggle_merge_request_redesign.yml

..._id_from_new_mr_dashboard_to_old_mr_dashboard_monthly.yml

..._id_from_old_mr_dashboard_to_new_mr_dashboard_monthly.yml

...r_id_from_new_mr_dashboard_to_old_mr_dashboard_weekly.yml

...r_id_from_old_mr_dashboard_to_new_mr_dashboard_weekly.yml

config/routes/dashboard.rb

locale/gitlab.pot

...erge_request_dashboard/components/action_dropdown_spec.js

spec/helpers/merge_requests_helper_spec.rb