Add bulk update support for merge requests list

app/assets/javascripts/dispatcher.js

app/assets/javascripts/issuable.js.es6

app/assets/javascripts/issues-bulk-assignment.js

app/assets/stylesheets/pages/issuable.scss

app/assets/stylesheets/pages/issues.scss

app/controllers/concerns/issuable_actions.rb

app/controllers/projects/issues_controller.rb

app/services/issuable/bulk_update_service.rb

app/services/issues/bulk_update_service.rb

app/views/projects/issues/_issue.html.haml

app/views/projects/issues/index.html.haml

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

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

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

app/views/shared/issuable/_filter.html.haml

spec/services/issuable/bulk_update_service_spec.rb