Allow Restricted Notebooks to export text (#6542)

* Refactor string to stream

* add to restricted notebooks and allow for blank users

* forgot to add notebook

* use better types and fix test

* move streamToString

* add export group

* catch blank pages

e2e/pluginFixtures.js

@@ -150,3 +150,17 @@ exports.test = test.extend({
     }
 });
 exports.expect = expect;
+
+/**
+ * Takes a readable stream and returns a string.
+ * @param {ReadableStream} readable - the readable stream
+ * @return {Promise<String>} the stringified stream
+ */
+exports.streamToString = async function (readable) {
+    let result = '';
+    for await (const chunk of readable) {
+        result += chunk;
+    }
+
+    return result;
+};