Retrieve audio feature data from frontend

Added user_secret field to User. Set up a basic JavaScript function for
querying audio feature data from the frontend.

spotifyvis/static/spotifyvis/scripts/user_data.js

@@ -0,0 +1,28 @@
+/**
+ * Retrieves data for a specific audio feature for a certain user
+ * @param audioFeature: the audio feature for which data will be retrieved
+ * @param clientSecret: the client secret, needed for security
+ */
+function getAudioFeatureData(audioFeature, userSecret) {
+    let httpRequest = new XMLHttpRequest();
+    /*
+     * Handler for the response
+     */
+    httpRequest.onreadystatechange = function() {
+        if (httpRequest.readyState === XMLHttpRequest.DONE) {
+            if (httpRequest.status === 200) {
+                let responseData = JSON.parse(httpRequest.responseText);
+                // TODO: The data points need to be plotted instead
+                for (let data of responseData.data_points) {
+                    console.log(data);
+                }
+            } else {
+                alert("There was a problem with the login request, please try again!");
+            }
+        }
+    };
+
+    let queryString = `/audio_features/${audioFeature}/${userSecret}`;
+    httpRequest.open('GET', queryString, true);
+    httpRequest.send();
+}