Zendesk Accordion Editor

This adds CSS into the Zendesk Guide WYSIWYG editor which causes the accordion headers to be rendered properly, and displays a light blue border around the content panel of the accordion. The accordions in the WYSIWYG editor intentionally do not collapse when clicked, and do not render section links or chevron icons. The light blue border will not be displayed on the public page once saved, it exists for reference purposes only. See the release notes: https://docs.google.com/document/d/1T8REHfz1a30_NC9sLdRTj_Jn-FjkENUFPcVOTDsMTjU/