CE upstream - Friday

Conflicts:

both modified:   app/assets/javascripts/vue_merge_request_widget/components/mr_widget_pipeline.js
both modified:   app/assets/stylesheets/framework/common.scss
both added:      app/models/epic.rb
both modified:   app/models/issue.rb
both modified:   app/services/issuable_base_service.rb
both modified:   app/services/merge_requests/update_service.rb
both modified:   lib/api/api_guard.rb
both modified:   spec/helpers/issuables_helper_spec.rb
both modified:   spec/requests/api/helpers_spec.rb