Merge branch 'remove-code-duplication'

.gitlab-ci.yml

app/controllers/concerns/issues_action.rb

app/controllers/concerns/merge_requests_action.rb

app/controllers/dashboard_controller.rb

app/controllers/groups_controller.rb

app/helpers/diff_helper.rb

app/helpers/gitlab_markdown_helper.rb

app/helpers/namespaces_helper.rb

app/helpers/selects_helper.rb

app/mailers/emails/issues.rb

app/mailers/emails/notes.rb

app/models/project_services/slack_service/note_message.rb

app/services/issuable_base_service.rb

app/services/issues/update_service.rb

app/services/merge_requests/update_service.rb

app/services/notification_service.rb

lib/gitlab/markdown/abstract_reference_filter.rb

lib/gitlab/markdown/issue_reference_filter.rb

lib/gitlab/markdown/merge_request_reference_filter.rb

lib/gitlab/markdown/snippet_reference_filter.rb