Factor out postToTokenUrl() function

Just separating concerns, and also future code is going to reuse this
function as well, reducing duplication.

src/sidebar/oauth-auth.js

@@ -40,18 +40,24 @@ function auth($http, settings) {
     };
   }
 
+  // Post the given data to the tokenUrl endpoint as a form submission.
+  // Return a Promise for the access token response.
+  function postToTokenUrl(data) {
+    data = queryString.stringify(data);
+    var requestConfig = {
+      headers: {'Content-Type': 'application/x-www-form-urlencoded'},
+    };
+    return $http.post(tokenUrl, data, requestConfig);
+  }
+
   // Exchange the JWT grant token for an access token.
   // See https://tools.ietf.org/html/rfc7523#section-4
   function exchangeToken(grantToken) {
-    var data = queryString.stringify({
+    var data = {
       grant_type: 'urn:ietf:params:oauth:grant-type:jwt-bearer',
       assertion: grantToken,
-    });
-    var requestConfig = {
-      headers: {'Content-Type': 'application/x-www-form-urlencoded'},
     };
-    return $http.post(tokenUrl, data, requestConfig)
-      .then(function (response) {
+    return postToTokenUrl(data).then(function (response) {
       if (response.status !== 200) {
         throw new Error('Failed to retrieve access token');
       }