Allow embedding HTML for external actions (#2693)

* Admin UI: implement HTML embeds

* Admin UI External Actions: set correct useHTML on edits

* Admin UI: edit by index, not URL

* External Actions: render HTML on stream frontend

* Don't open embeds externally

* Remove TODO comment

* Add HTML as unique action key

* Admin UI: Actions: use CodeMirror editor, dropdown

web/interfaces/external-action.ts

@@ -2,7 +2,8 @@ export interface ExternalAction {
   title: string;
   description?: string;
   color?: string;
-  url: string;
+  url?: string;
+  html?: string;
   icon?: string;
   openExternally?: boolean;
 }