Refactor AuthService

Refactor AuthService to be a more conventional class with instance
methods and fields as per other services.

Make a small change to the API service to ensure that `getAccessToken`
is correctly invoked on an auth service instance.

src/sidebar/services/api.js

@@ -217,7 +217,7 @@ export class APIService {
     /** @param {string} route */
     const apiCall = route =>
       createAPICall(links, route, {
-        getAccessToken: auth.getAccessToken,
+        getAccessToken: () => auth.getAccessToken(),
         getClientId,
         onRequestStarted: store.apiRequestStarted,
         onRequestFinished: store.apiRequestFinished,

src/sidebar/services/test/auth-test.js

@@ -137,7 +137,7 @@ describe('AuthService', () => {
     context('when the access token request fails', () => {
       const expectedErr = new Error('Grant token exchange failed');
       beforeEach('make access token requests fail', () => {
-        fakeClient.exchangeGrantToken.returns(Promise.reject(expectedErr));
+        fakeClient.exchangeGrantToken.rejects(expectedErr);
       });
 
       function assertThatAccessTokenPromiseWasRejectedAnd(func) {