Show what time ago a MR was deployed

CHANGELOG

app/assets/javascripts/merge_request_widget.js.es6

app/assets/stylesheets/pages/merge_requests.scss

app/controllers/projects/merge_requests_controller.rb

app/models/environment.rb

app/models/merge_request.rb

app/models/repository.rb

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

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

spec/features/merge_requests/widget_deployments_spec.rb

spec/javascripts/merge_request_widget_spec.js

spec/models/environment_spec.rb

spec/models/repository_spec.rb