Create Danger rule to ensure frontend tests have been added

Create a Danger rule that displays

• an error if new JavaScript files are added without tests
• a warning if JavaScript files are modified but not the corresponding tests
• a warning if new Karma tests are added (instead of using Jest)

This implicitly makes sure we don't end up with inconsistencies like

• app/assets/javascripts/vue_merge_request_widget/
• spec/javascripts/vue_mr_widget/