Render and sanitize chat messages server-side. (#237)

* Render and sanitize chat messages server-side. Closes #235

* Render content.md server-side and return it in the client config

* Remove showdown from web project

* Update api spec

* Move example user content file

webroot/js/web_modules/import-map.json

@@ -6,7 +6,6 @@
     "@videojs/themes/fantasy/index.css": "./@videojs/themes/fantasy/index.css",
     "htm": "./htm.js",
     "preact": "./preact.js",
-    "showdown": "./showdown.js",
     "tailwindcss/dist/tailwind.min.css": "./tailwindcss/dist/tailwind.min.css",
     "video.js/dist/video-js.min.css": "./videojs/dist/video-js.min.css",
     "video.js/dist/video.min.js": "./videojs/dist/video.min.js"