message: Add IS access token callback
  This passes a callback to the JS SDK which it can use to get IS access tokens
  whenever needed for either talking to the IS directly or passing along to the
  HS.
  Fixes
diff: @@ -32,6 +32,7 @@ import Modal from './Modal';
  import {verificationMethods} from 'matrix-js-sdk/lib/crypto';
  import MatrixClientBackedSettingsHandler from "./settings/handlers/MatrixClientBackedSettingsHandler";
  import * as StorageManager from './utils/StorageManager';
  +import IdentityAuthClient from './IdentityAuthClient';
  interface MatrixClientCreds {
  homeserverUrl: string,
  @@ -219,6 +220,9 @@ class MatrixClientPeg {
  fallbackICEServerAllowed: !!SettingsStore.getValue('fallbackICEServerAllowed'),
  verificationMethods: [verificationMethods.SAS],
  unstableClientRelationAggregation: true,
  + getIdentityAccessToken: () => {
  + return new IdentityAuthClient().getAccessToken();
  + },
  };
  this.matrixClient = createMatrixClient(opts);
  