Add a logged in class to EmbeddedPage and react to MatrixClient changes

See https://github.com/vector-im/riot-web/pull/9957

The two hacks introduced here are for different reasons, mostly related to the welcome page. If you land directly on the welcome page, the app's lifecycle is highly unlikely to have a bootstrapped client. This results in the loggedIn class being false. When the client is later set up (loaded from session, new guest account registered, etc) the context fails to update for the EmbeddedPage, and we need to give it a kick to re-render. It's arguable if we should even keep using the context here.

src/components/structures/EmbeddedPage.js

@@ -24,6 +24,8 @@ import request from 'browser-request';
 import { _t } from '../../languageHandler';
 import sanitizeHtml from 'sanitize-html';
 import sdk from '../../index';
+import dis from '../../dispatcher';
+import MatrixClientPeg from '../../MatrixClientPeg';
 import { MatrixClient } from 'matrix-js-sdk';
 import classnames from 'classnames';
 
@@ -82,19 +84,31 @@ export default class EmbeddedPage extends React.PureComponent {
                 this.setState({ page: body });
             },
         );
+
+        this._dispatcherRef = dis.register(this.onAction);
     }
 
     componentWillUnmount() {
         this._unmounted = true;
+        dis.unregister(this._dispatcherRef);
     }
 
+    onAction = (payload) => {
+        // HACK: Workaround for the context's MatrixClient not being set up at render time.
+        if (payload.action === 'client_started') {
+            this.forceUpdate();
+        }
+    };
+
     render() {
-        const client = this.context.matrixClient;
+        // HACK: Workaround for the context's MatrixClient not updating.
+        const client = this.context.matrixClient || MatrixClientPeg.get();
         const isGuest = client ? client.isGuest() : true;
         const className = this.props.className;
         const classes = classnames({
             [className]: true,
             [`${className}_guest`]: isGuest,
+            [`${className}_loggedIn`]: !!client,
         });
 
         const content = <div className={`${className}_body`}