An example Discourse plugin for whitelisting iframe urls
Note: this plugin is broken for the most recent version of Discourse. You can now whitelist iframes sources through the Discourse admin settings page.
Fork this repository and add the urls that you wish to whitelist to assets/javascripts/lib/discourse-markdown/whitelist-iframe.js.es6.
For older versions of Discourse, you may need to add the whitelisted urls to assets/javascripts/iframe-whitelist.js instead.