Simplified scheduling-auth-token module's interface

refs https://github.com/TryGhost/Team/issues/694

- Only passing necessary data into the module simplifies it's interface and allows to decouple it further from model layer dependencies
- Note, also verified and corrected the return type of the auth token creating method

core/server/adapters/scheduling/post-scheduling/index.js

@@ -17,9 +17,17 @@ const SCHEDULED_RESOURCES = ['post', 'page'];
  */
 _private.normalize = function normalize({model, apiUrl, resourceType, integration}, event = '') {
     const resource = `${resourceType}s`;
-    let publishedAt = (event === 'unscheduled') ? model.previous('published_at') : model.get('published_at');
-    const signedAdminToken = getSignedAdminToken({publishedAt, apiUrl, integration});
+    const publishedAt = (event === 'unscheduled') ? model.previous('published_at') : model.get('published_at');
+    const signedAdminToken = getSignedAdminToken({
+        publishedAt,
+        apiUrl,
+        key: {
+            id: integration.api_keys[0].id,
+            secret: integration.api_keys[0].secret
+        }
+    });
     let url = `${urlUtils.urlJoin(apiUrl, 'schedules', resource, model.get('id'))}/?token=${signedAdminToken}`;
+
     return {
         // NOTE: The scheduler expects a unix timestamp.
         time: moment(publishedAt).valueOf(),
@@ -58,6 +66,7 @@ _private.loadScheduledResources = async function () {
 /**
  * @description Initialise post scheduling.
  * @param {Object} options
+ * @param {string} options.apiUrl -
  * @return {*}
  */
 exports.init = async function init(options = {}) {

core/server/adapters/scheduling/post-scheduling/scheduling-auth-token.js

@@ -4,11 +4,16 @@ const jwt = require('jsonwebtoken');
 /**
  * @description Get signed admin token for making authenticated scheduling requests
  *
- * @return {Promise}
+ * @param {Object} options
+ * @param {string} options.publishedAt - ISO date
+ * @param {string} options.apiUrl - url of the JWT's audience
+ * @param {string} options.key - integration key
+ * @param {string} options.key.id - key ID
+ * @param {string} options.key.secret - key secret
+ *
+ * @return {string} the JSON Web Token
  */
-const getSignedAdminToken = function ({publishedAt, apiUrl, integration}) {
-    let key = integration.api_keys[0];
-
+const getSignedAdminToken = function ({publishedAt, apiUrl, key}) {
     const JWT_OPTIONS = {
         keyid: key.id,
         algorithm: 'HS256',