Adds rel="noopener noreferrer" to all links with target="_blank"

See merge request !2071
Conflicts:
	app/assets/javascripts/environments/components/environment_external_url.js
	app/assets/javascripts/merge_request_widget.js
	app/helpers/commits_helper.rb
	app/views/projects/services/mattermost_slash_commands/_detailed_help.html.haml
	app/views/projects/services/mattermost_slash_commands/_help.html.haml
	app/views/projects/services/slack_slash_commands/_help.html.haml